My lil brother just clicked a whole load of boxes in a extensions off the main menu it had things like Tf2 and now its not letting me play without down loading like 6 other games is ther any way i can change it back???
i cant Get INTO GMOD.

If GMod will not open whatsoever, uninstalling and re-installing will work.
You could try deleting your garrysmod/garrysmod folder before that, though.