Many "free" GMod accounts are actually . A previous user used hacks on a server, and now the account is permanently banned from secured servers. If you log into this account, you inherit the VAC ban. Furthermore, Valve tracks "ban evasion." If you attempt to play on a new account from the same PC, you risk a ban there too.
Searching for "free Steam accounts with Garry's Mod (GMod) " often leads to fraudulent websites, security risks, or account bans. According to Steam Support Free Steam Accounts With Gmod
GMod 13 (the version before the current one) is sometimes given away for free on modding forums for archival purposes. It lacks multiplayer support and modern features, but for pure single-player sandbox building, it's a legal, free alternative. Many "free" GMod accounts are actually
Keep an eye on Steam's store for free trial periods of games, though Gmod's base game does require purchase. Furthermore, Valve tracks "ban evasion
This article will dissect exactly what you are risking when searching for these accounts, and most importantly, provide legitimate ways to play GMod for free or cheap.