You could either install it yourself (add new game, runner wine steam) or create an installer for it. For both of it you need the Steam App ID, which you get by simply visiting its Steam page (see URL).
If you create an install script, open any other Steam game install and also your game’s GOG installer. Compare, if settings are missing, otherwise you just need the App ID
He was talking about creating the actual game page, which has to exist in order to create installers.
@stooj if you don’t have the game on Steam, it’s ok to create the page but notify a moderator so they can fill in the Steam ID field and we don’t end up with duplicates when someone syncs their library with this game.