sm_gravity is a command to specifically target a client. For example:
sm_gravity Player1 200
If you want to change the whole servers gravity, use:
sm_cvar sv_gravity 200
Most plugins do not automatically add anything to the admin menu, you will need to custom edit it yourself to add the various plugin commands you want.
Your crashing problems could be due to you not fully installing SourceMod. Make sure that you uploaded the entire archive to your server. You could be missing some offset files which are required.