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Leroux: I don't know either, but one difference seems to be that on Steam the devs can handle everything on their own, while on GOG they have to communicate with GOG's staff for everything and depend on the staff for e.g. when GOG will test the game/DLC/update, when they will create an installer for it, when they will put it up etc.
Hasn't this been improved lately? I thought developers could now upload their builds directly to Galaxy, with GOG taking care of the installers. Some of the effort involved in needing to make the game not rely on Steam features has also been taken over by GOG.

For me, the point of departure for any developer is that every customer buying the official product is treated equally. That's how any company operates within a professional capacity. Buying the product at a different store has no bearing on the required support. The expectations of game developers in regards to selling their products can at times strike me as puzzling.