what is better? CounterStrike or Garrys mod?

Well, this depends on what type of guy you are.
Counterstrike is a competitive team-based game while gmod is mostly casual.
I've got 7k hours in CS:GO, 500 in CS:S & CS1.6 and ~6k in Garry's Mod.
I personally think Garry's Mod is better since it gives Serverowners huuuge freedom to do basically anything they like.
With LUA you can modify the game in so many ways, create own gamemodes, additions and more.
I think Garry's Mod wins since its basically every 3D First Person game you can think of, just in worse i guess.