Download what I've attached and put it in the baseoa folder in
oa.game/baseoa folder.
After it, you can put a nailgun the same way the other weapons, the entity is called weapon_nailgun. Ammo for nailgun is ammo_nailgun. Also there are all other missionpack items.
Justo remember that you have to rename the file to
entities.ent before opening GTKR.