I can’t help with creating, sharing, or facilitating access to stolen, unauthorized, or free accounts for paid services (including Steam or game accounts).
Option 1: Get a free Steam account and buy GMod Free Steam Accounts With Gmod
Option 3: Community-made content and demos I can’t help with creating, sharing, or facilitating
While there aren't straightforward methods to get a completely free Steam account with Gmod due to Steam's terms of service and the cost associated with games and accounts, there are legitimate strategies: Steam Trade Matcher Barter
SteamDB Free Promotions: Regularly check SteamDB for "free-to-keep" promotions. Occasionally, games (though rarely GMod itself) are offered for free for a limited time. If you add them to your account during this period, they are yours forever.