Matchmaking is nothing more than a user database query. That database sits on GOG’s servers and the only thing GOG does is put users into a lobby and then send that data back to the clients so the game can show it to the user. And then when the game starts GOG connects the clients to the host. So developers don’t have to setup their own lobby and relay server. That host can be another client, then the developers don’t have to pay for anything, or a dedicated server which the devs have to pay for themselves. And in case of a dedicated server clients do not have the server binaries.