I go to the Mod menu from the main menu and when I select "Install" with a mod, I get a pop up about mod.io, where I have to enter my email. After that, nothing happens. I can't get rid of this pop up, no mod gets downloaded, nothing happens on mod.io.
Only way to exit it by ALT+F4 out of the game...

How does this work on gog, please?

Officially, you are supposed to sign up with mod.io and have them handle it.

Unofficially:
Step 1) You are on your own.
Step 2) nexusmods (an outside website with no affiliation what-so-ever) and hope the uploader provided instructions and keeps the directions updated.

{QAD for this games DLCs:
Step 1) Hover your mouse over your user name in the top bar of the gog.com website.
Step 2) Select Games
Step 3) Hover mouse over lower right corner of "The Riftbreaker" card and a drop down menu option will popup. Click the ▼ button to trigger pop out menu.
Step 4) Select "View Downloads".
Step 5) You will see a list of DLC packages. Download and install to taste. [GOG Galaxy will keep them up to date. (If desired.)]}

Running into the same issue myself, installing mods by adding to the game folder doesn't seem to work anymore either.
I've gone and downloaded mod files manually from the mod.io site and tried putting them into the mod.io folder located in the public folder and still can't get it to work. If ya happen to come across something let us know!

**Edit: My most sincere apologies to the entire staff behind The Riftbreaker.

https://riftbreaker.fandom.com/wiki/Modding

Yeah, I fixed it by returning it on GoG and buying it on Steam.

Well this should not be the #1 forum thread. The issue is resolved and everything works great now.

(btw thanks for the great game!)

Whether this is resolved or not, installing mods for the GoG version of The Riftbreaker is the simplest thing ever (it was actually so easy, it confused me at first).
Download the desired .zip-file from Nexusmods, and place that .zip directly into:

[X]\The Riftbreaker\packs

Do NOT unpack it, or touch the .zip in any way!

Nope packs folder does not work anymore. And mod.io registration is unresponsive as well. Could be because i will not give my real email, but how would they know?

Why the **** did they think it a good idea to change everything mod-releated and document nothing(useful anyway).

I even tried to "fake" the resource id to get the nexus-mod loaded, but no it does not work either.

It seems i am not alone either:
GOG will not let me post a ****ing link.

(hypertext ssl prefix) + "forums" + ".nexusmods.com/topic/13470863-all-the-riftbreaker-mod-no-longer-work/"

This one, I assume.
https://forums.nexusmods.com/topic/13470863-all-the-riftbreaker-mod-no-longer-work/

Curious how almost all mods ceased at around 2021.

My guess is the game was rewritten, somehow, to accept mods in a different way.
I have submitted my email to mod.io, and those mods work. Though that defeats the whoel purpose of "offline play", if I need an internet connection while in the game menu to download and install mods...

So yeah, not pleased about this change.

According to https://www.nexusmods.com/theriftbreaker/mods/3?tab=posts, they changed the mod directory in one of the patches. Apparently, mod files need to go in a subdirectory called "mods" instead of the "packs" which is listed here.

I can't test it right now, as I uninstalled the game when I finished playing it to save space on my SSD.