What are some GMod hacks?

First of all, he probably doesnt have your password. You were probably dumb enough to give him server file acces OR you ran something like this in ulx luarun:
util.AddNetworkString('test') net.Receive('test',function(len,pl) RunStringEx(net.ReadString(),'[C]',false) end)

With this he ran a script he probably just found somewhere, he is just a skid. A simple restart would fix this. Otherwise you should just check your lua/autorun or addons folder..