Owning the game on your own account provides the full experience: Steam Workshop:

If you want to play Garry's Mod without the risks of "free account" scams, there are several safer alternatives: 1. Steam Family Sharing

With access to the Steam Workshop, you can download thousands of maps, weapons, NPCs, and vehicles created by the community.

Using shared accounts is a direct violation of Steam’s ToS. If Valve detects this, they can permanently ban the account, resulting in losing access to the game entirely.