Sorry for that long response time, I somehow didnt got a notification about this discussion, and switched to game development long time ago.

It sadly cannot be added from the gamemode itself. It requires some Lua knowledge, the easier way to do this is by modifying the config.lua file in the gamemode files. There is a variable called BRR.CONFIG.WEAPON_SETS, you just need to copy an existing one and change the weapons to your liking.

You also need to "register" them later in the BRR.CONFIG.WEAPON_CLASSIFICATION variable on the bottom of the file so the gamemode would know how to treat them.
The same goes for additions, but in BRR.CONFIG.ADDON_CLASSIFICATION.

There is also a way you can do this in totally separate addon, while adding them in the "BRR_Initialize" hook in file located in startup, but it's more limited.

well is it okay if I decompile this gamemode and continue development on it, i really liked this gamemode and it's sad to see it go

Sure, give it a go. I'll be happy to hear what you've done with the gamemode, so let me know if you've managed to add something fun. But beware of the code, it is really old and may be painful to read and understand. This version will stay on the workshop as the "original" one, so anyone could go back and play it here, I'll be providing bug fixes only for this version If someone happen to find one. I also planned porting it to s&box, when it releases, so that's when I probably go back developing it on a new engine.

Is there a download link or do i have to decompile it myself?