This was just added in the latest version (2020.08.12). It might only be available on the Dev Branch right now.
Returns the 64-bit SteamID aka CommunityID of the Steam Account that owns the Garry's Mod license this player is using. This is useful for detecting players using Steam Family Sharing.
If player is not using Steam Family Sharing, this will return the player's actual SteamID64().