Greetings fellow players,
I need some help figuring something out...
I bought Quake: The Offering as well as Quake II: Quad Damage from GOG but the shortcuts launche only the base games so I browsed into the mission pack folders but I couldn't find any executable so how do I play the mission packs please?

In Windows there should be 3 shortcuts
(e.g. Win7) > All Programs > GOG.com > Quake - The Offering >
1. Quake (base game, miniGL 0.95)
2. Quake - Scourge of Armagon (Mission Pack 1, miniGL 0.95)
3. Quake - Dissolution of Eternity (Mission Pack 2, miniGL 0.95)

You may also create 2 "proper" shortcuts or edit existing to run correctly
glquake.exe -hipnotic -game hipnotic (for MP1)
glquake.exe -rogue -game rogue (for MP2)

I suggest to find and download GLQuake 0.97 or 0.98a version (glquake.exe file) or better yet use FitzQuake Mark V or QuakeSpasm source engines for better compatibility (Google it).
You'll also need to find and download or to extract, deemphasize and correct cue sheets into MP3 or OGG files for best experience. Due to legal issues current GOG version of GLQuake have no music.

P.S. For Linux based OS make sure that folders id1, hipnotic and rogue are all lowercase.

Greetings,

I already solved the problem, I use Quakespasm thanks to the advices from PCGamingWiki: https://pcgamingwiki.com/wiki/Quake
I copied the folders "id1", "hipnotic" and "rogue" from the Quake folder, pasted them into the Quakespasm folder then I just launch Quakespasm and use the console command "game hipnotic" or "game rogue" or "game whatever-expansion-folder-name" to play whatever expansion pack. ;-)
Also, I downloaded the de-emphasized soundtracks from here:
Quake -> https://www.quaddicted.com/files/music/quake_campaign_soundtrack.zip
Scourge of Armagon -> https://www.quaddicted.com/files/music/quake_mp1_soundtrack.zip
Dissolution of Eternity: https://www.quaddicted.com/files/music/quake_mp2_soundtrack.zip
So now I hear good quality soundtracks when I play with Quakespasm. :-)

Actually, according to PCGamingWiki, only the DOS version of Quake and mission packs play their soundtracks out of the box in the GOG version.
I don't understand legal issues which make GOG forbidden to offer a ready to play out of the box game while allowing to include soundtracks in files...
I don't understand how does work the mind of the strange beings who set the rules.
But even so, original Quake engines are only able to play soundtracks from a CD drive, being physical or virtual, so a source port is needed in order to play soundtrack files from the hard drive anyway.
I am happy that modders exist. :-)

Then about Quake II, I use KMQuake2 thanks to the advices from PCGamingWiki: https://pcgamingwiki.com/wiki/Quake_II
In order to play mission packs with KMQuake2, the solution is there: https://www.gog.com/forum/quake_series/quake_2_mission_packs_with_retail_quake_2_and_kmquake
And about the soundtracks, all that we have to do is to copy the music folder from the Quake II folder then paste it into the "baseq2" folder and we are ready to play with KMQuake2! ;-)

There is also this option for those who don't wanna resort to mods or alternative engines.

Just wanted to thank you for this.

And what about those who don't want to resort to a client app? :-p

Why not? For this is good solution. Game remain drm-free in GOG Galaxy (except very few titles).

For Quake 2 game+addon+mods started by default GOG link from optional GOG Q2 Launcher. It possible to add there more mods or replace main executable. Im never check Quake 1 (never be a real fan), as i can see there is new launcher exe file, but there is no config for it. If you want easy solution - it be GOG Galaxy (require Win8.1 but barely work o Win7). Or some 3rd party.

If no client apps at all - manually created links. Just like in good old days. Go ahead, you are free to choose. :p

I think GOG Galaxy is not ready yet. I tried it a few times and every time, it caused troubles with my games.
I don't mind an app who makes my life better, but if it actually makes my life harder, no thanks, polish it before I use it!
That's why I stick with the good old way for now, I just download my games from GOG then I manage them myself.
Moreover, why would I choose to stick with the outdated restrictive original game engine when there are some upgraded alternative engines who expand the possibilities freely available?
Playing Quake with Quakespasm and Quake 2 with KMQuake 2 doesn't break the original spirit, it merely remove the technical limitations from an outdated technology to allow the games to achieve more than they could do at the time of their creation.
Anyway, an upgraded engine is required to play the fan-made mission packs who are too much for the original outdated engine, just leave it to rest in peace, it did its job, now it is no more needed, it can rest and leave it to its successors. ;-)

Yamagi Quake 2 limits itself to bugs fix, it sticks to the original limitations from 1997, which has become irrelevant nowadays, KMQuake 2 allows Quake 2 to give its best while sticking to the original feel.
You have to understand that breaking the limits of the original engine doesn't mean breaking the original spirit because those original limits were not from intended design but from technical limitations from outdated technologies.
Upgraded engines merely allow games to achieve what their original designers dreamed of but couldn't because of the limitations from their time.

And about GOG Galaxy, I already explained why I don't want to use it: it messes with my games! It causes troubles that are not there when I don't use it.

I agree for Galaxy. I almost not used GOG Galaxy too. Witcher only. Testing. And since Win7 is not supported, Galaxy barely work on it - bad testing. Im still use Steam as a launcher for games. I do used it cause it more easy to set "compatibility" (basic functions adminrun, disablethemes x2, disable DEP x2, disable maxwindowedmode for Win8+), core affinity, priority to every games by one Win Registry edit. And FPS counter, and sometimes chat\overlay browser.

I prefer offline installers and DRM-free games. I do not use GOG Galaxy for Quake 2 (well, it already have useful launcher that work like configurable bat-file, no any else needed), and if im play Q1 - still be offline version.

But technically GOG Galaxy allow to configure executable for games add additional links to start not-\modded games. Steam not allow that.
There is another useful tools, indendet to be replacements for desktop links. Or tools made for something else, but can be used as unifiq launcher - i sometimes use DxWnd tool for it. No hacks enabled usually (except FPS counter if it possible for game render), or some enabled if it old directdraw render not so good worked on Win7.

Also DxWnd allow emulate AudioCD from Win98 that work differ back then (each new Windows lose something from Sound Mixer \ setting, now Win10 even lose with another update builds), disable maxwidnowedmode for Win8 (every directdraw\direct3d games prior to d3d9 affected) and many else.

AudioCD volume should be changed for Quake 1 by DxWnd. For vanilla i mean. Should be also for Quake 2, but OGG now lose separate music volume, for every audiocd->ogg digital releases separate music volume is entirely broken and cannot be fixed. This is why it better bundle game also with original disc image.

DxWnd can fix "security update" from Win7+ that disable audiocd ingame entirely - one track per loading. Quake 1-2 affected by this security. Quake 1 used disc image still affected even in GOG DRM-free digital release. But it enough action there, so fast, so much loadings - music almost looped. And even such AWESOME soundtrack - i dont like it looped for Quake 2.

I prefer clean original games. And Quake 2 require to be played in some source-port. There is no limitation - devs decide to do every render own unique features. Even for software mode. Even 3dfx render is not superior.
Yamagi fix everything it should be fixed, remove those pseudo-limitations from devs. Same as KMQ2.
Yamagi as one of the vanilla-est sourceport-mod. I know Quake 2 RTX based on some sourceports-mod's, ofc VPK-one is not vanilla-like. Cant say much for opengl one. But Quake 2 RTX launched with -gl argument in OpenGL mode is same vanilla-like as Yamagi.
More lite- and -vanilla like sourceport-mods, that fix "limitations" by devs - Facelift-mod and DirectQII. There is also DirectQII 11 on github, i didnt test it. D3D9 is enough for me.

I dont trust KMK as author. Entirely. Every his work. And Quake 2 is a reason for that. Sourceport-mod, "patch 3.24" (bruh) and... even when "patch" exist - there is also lite-sourceport-mod from him doing same things. This is unnormal for modder, show how he understand what he doing. It just experience with opensource for him, and not serious work. If limit me in words.
It just like downloading Steam Community guides content before steam guides exist.

For more improvements and changes to graphics and gameplay, there is better sourceport-mods. or not better... some changes... But that always questionable. This is why original always stay better.
Q2XP, Q2VPK\RTX, berserker, Evolve, ... which ones else?

I wish good port from Quake 2 to Quake 4... dreams, dreams...
Also, Quake 2 Xbox360 - best version probably. Emulator + PAL disc image... dont know where to get good disc image... It also previously be available in digital format for Xbo360.
Anything modern consoles are good - ports games from PC. New games, especially are from consoles - trash\blured-soaped-movie-trash.

UPD: Oh yeah, i forgot something else like Facelift-mod (made for fixing smothed rendering for opengl that made by devs for advertising powervr\3dfx) - scaled-up textures packs. Should be one of that on Steam Community game group guides page and on moddb... ... I see two differ texturepack mods on moddb.