myconv: I admit I don't know how Steam would react, it's yet to be an issue since almost everything I've played run fine on Proton (sometimes I have to research particular launch commands to put in to get things working) Maybe I've just been ultra lucky with tens of games non-native to Linux?
But in the end it doesn't matter. You have customers with a need directly related to a product you sell. You can hire people to fill that need. Otherwise all your doing is hosting the download and not providing any support for some of your customers. Can they just refuse to support some of their customers? Yes, But should they? Are any of you seriously arguing for such? If so, why? What benefit is it to you to tell me off, presumably on GoGs behalf, even though it doesn't benefit them.
Also as mentioned before it seems GoG has a more direct connection to the problem using a nonstandard window registry for Civ 4.
If 2K Games does not want to provide a Linux version of Civ 4 then GOG can not do anything about it. If you have an issue with no Linux version of Civ 4 then you need to talk to the publisher as GOG can not do anything legally to provide support or offering a Linux version to a game they only allowed to sell for the platform the owner has specified.
You can't just say "Okay they can't offer support since the game is not available for Linux, but they should still offer support for Linux".
The only way to get an official version of Civ IV on Linux is to contact the publisher and request it.