Some people who have repliedare misinformed about the VAC system and how it integrates through Garry's Mod. VAC at one point WAS a possibility on Garry's Mod, and still IS but your chances of being VAC'd are extremely slim.
You see, most """""Hacker""""" on Garry's Mod run lua cheats, where they bypass the servers anticheat (if it has one) and just mess around with clientside lua which is the type of cheating that is IMPOSSIBLE to become VAC'd by. Valve Anti Cheat is designed to detect changes in the games base code and there are very few files in the Garry's Mod root file that you may edit in order to receive a VAC ban, but I'm pretty sure these were patched up and fixed around to avoid such a situation from occurring. Just like any other game on the Valve network, you can get VAC banned for fiddling with core files that give you an advantage on ANY server but it's extremely unlikely that you'll suffer any consequences from it.
ALL Garry's Mod servers that run via SRCDSare VAC secured, just like any other source game.