No matter where to choose to install the racks, they must be installed into the wall studs or supports. Drywall, by itself cannot support the racks. The wall support could possibility be wood 2x4's, 2x6's, metal studs, steal beams, cement walls, cinderblock, etc. You must make sure all the mounting hardware gets secured into these supports.
Also Very Important
Gallow rack system MUST be installed into a secured location. This way your guns remain safe and locked up away from anyone that should not have access to them.