This mod from dakka_perkele00 makes the GP37 more expensive and have more ammo.
GP 37 REWORKED
Just put the content into your gamedata folder
Gp 37 now has 300 round clip and cost's 18 rubbles
(+grenade laucher and silencer, but not from me)
Credit goes for SiriusStarr for his "GP 37 GRENADE LAUNCHER MOD"
You dont have to start a new game
You can use this in your mod, just give a little cred if you want.