On the contrary. GOG.com just want to make sure that when it commits to Linux support, it would be able to support all of the major distributions. And they are looking for ways to do that. Here's an interview where they expanded upon it a little:
Originally Posted by Larian
It's a bit unfair towards Valve, as they did say they would also support other distros, but otherwise it makes sense. After all, for all of the DOSBox games, you can already play them on Linux without issues. Just extract their installer and that's all. It's only the native Linux games that are not available yet.