Whenever I put on auto join to the server my gmod crashes on joining, i've been trying to join the server for three hours but I can't join the normal way because it's full.
That sounds very strange indeed, have you tried configuring your Garry's Mod settings and launch options? You can also try and verify the integrity of your Garry's Mod files: Library > Right-click Garry's Mod > Properties > Local Files > Verify Integrity of Game Cache.
Let me know if that solves it, if not, a clean install of Garry's Mod should also do the job.