Player:OwnerSteamID64
Description
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().
This data will only be available after the player has fully authenticated with Steam. See Player:IsFullyAuthenticated.