From what I know, you can't. The only way you can add a gun is if it is implemented in the RC(Remote Control), which only staff have access to. If you are just starting ganis, I would recommend you practice on the offline editor. Because eventually you will need it if you ever decide to apply for a staff position. Obviously the offline editor is harder because there is not sprite cutter, rotate, etc. But that is something all of us Gani Artists should master. Good luck!