Im going to copy my post from another thread because its the same question and so people can stop asking about the existance on VAC-bans in Garry's Mod.
Garry's mod itself doesnt have Valve Anti Cheat, but most servers use individual anti cheats that detect the more modern hacks.
You can only get banned on individual servers or servers owned by the same group, Gmod also does not restrict players with former VAC bans from playing multiplayer.
But Bigpackets cant be detected by these newer anti-cheats making it hard for servers to stop people from hacking (atleast people using BP)
relying on "admins" who actually care about the server constantly punishing people who are obviously hacking.
But honestly most Gmod servers have pretty bad staff and its easy to hack without being noticed because most admins wont even spectate you if people are accusing you of cheating.
However be careful of staff not on duty if you are playing with hacks, if you're "too" good they will accuse you of hacking and will try to get you banned (or ban you if high enough position) based on a flimsy hunch.
"Badmins" are your friends when you're hacking because less capable admins mean higher chance of using obvious hacks without getting punished in any way shape or form.