I've spent a fair amount of time on the internet looking for GSC related materials, and here I will share all the resources I reference when I get stuck on something.
Most WaW and BO1 era scripts are compatible with BO2 since GSC wasn't altered much between WaW and BO2.
BO3 however is much different from earlier games scripting and it requires more effort to backport scripts to BO2
CabConModding doesn't require an account to view scripts:
CabConModding also has many open source mod menus that can be useful as examples.
UGX for more generic COD related scripting but still useful:
The main site for UGX requires an account to view links and scripts:
the Zeroy wiki containing a lot of information for different CODs I recommend looking at WaW and BO1 scripting sections for BO2 related scripting:
ZombieModding requires an account to access but many scripts are available to study and utilize:
There may be more sites that I am not aware of but if anyone else has any please do share and I will edit the post with them.
Studying how the actual game works, in gsc, is very useful as well.
And also allows you to do all kinds of cool stuff, like replace some behavior when something happens, or maybe re-use some of the gsc that's already in the game so you don't need to custom make everything.
The stickied post in here has a download to a gsc dump. Then also get get some program that searches for text from inside multiple files. Useful when you're looking for something specific, or want to see where some function is defined, where it's called, etc etc etc. Been using "AstroGrep" myself to do this.