GodotSteam 4.17 through 4.23.2 trusts the peer value of 1 to mean that a player is the server. A grant_gold(1000000) call can be forged.
A Godot game whose multiplayer transport is GodotSteam's SteamMultiplayerPeer (create_host, host_with_lobby, or connect_to_lobby), in a session the attacker can join: a public or joinable lobby, matchmaking, or an open dedicated server. Games using only achievements, input, or leaderboards, and games on the ENet or WebSocket transports, are not exposed.
Game-side only: on the plugin's peer_connected signal for peer ID 1, compare Steam.get_steam_id_for_peer_id(1) with the expected server Steam ID (the lobby owner, or the ID passed to create_client) and close the multiplayer peer on a mismatch. The guard ships in the reproduction kit. No plugin setting rejects a false claim.