In the mood for some games that let me do some weapon customization I.E. CoD Warzone or Tarkov, the only game that comes to mind on GoG is the Metro games, any games notable on the platform for deep weapon customization?

Don't know if it qualifies as deep, but S.T.A.L.K.E.R. Clear Sky and Call of Pripyat had weapon customisation.

What do you mean by "deep" weapon customization? Lot of options? Or lots of impact on gameplay, even if the options are not that many?

For instance CP2077 lets you "mod" weapons. One of the mods with the biggest impact is to make them non-lethal.

i don't know what kind of weapon customization those games have that you mentioned but i remember stalker clear sky and stalker call of pripyat had weapon customization that can increase range, clip and other things.
Also Hellgate London also had that but this is not on gog.
And i don't even know if the steam version runs properly since i have the retail version of Hellgate: London

like to adapt to different roles or just to screw around, like choosing between a red dot and short barrel for CQC or a 8x scope and a long barrel for longer ranges, less straight upgrade and more side grades/situational builds.

IIRC Fallout New Vegas had weapon customization.

basically a game that lets you customize to the extent close to this video

https://youtu.be/rWg4Fxx2GyA

I don't remembering it having too many options per weapon, but there are probably mods that dramatically expand upon it, so I will probably look for some mods for NV.

Warzone 2100 has a nice custom unit designer. You can even recycle old units but keep the experience.

was thinking more FPS than anything but you mentioning that brought me back to high school and I got to install that game again lol.

MOTHERGUNSHIP - https://store.steampowered.com/app/574090/MOTHERGUNSHIP/

There's a long list of games that allow you to customize your weapon to some extent... The Sniper: Ghost Warrior series include some decent weapon customisation, for example.
But my favourite game in that aspect is Hitman: Blood Money.

You'd probably be surprised to know that there's a new version.

Polaris Sector knows a wicked design feature, be prepared to be mauled at all sides in the first few games....

also Star Sector is pretty satisfying with what it offers on the customization level, a bit more easier on its users too

3d person action games such as Mass Effect Andromeda and Outriders of course also score quite well on the customization department

The Elder Scrolls: Morrowind allows you to enchant weapons with custom enchantments. Oblivion also does there. (Also, Daggerfall, though that particular game's status on GOG is a bit strange; it's treated as a delisted game (you can't buy it), but buying any Bethesda game on this site (not just a TES game!) will add it to your library.)

Elminage Gothic allows you to modify weapons via alchemy; you just need to find the right material, have an Alchemist in your party (level 1 is enough IIRC, and you do this in town, where you can freely change your party), and you can give your weapons and armor the properties you choose. (For example, health regen on armor is handy early in the game.)