DaCostaBR: I imagine that most people willing to buy it at full price have done it already on Steam.
I think you're underestimating the GOG cultists.. I mean customers.
Personally I would probably still prefer the Steam version (or a Humble Store sale with both DRM-free and Steam) unless the GOG version includes the soundtrack and the Steam version doesn't (too lazy to check).
Assuming it is Guacamelee, it is probably the developers fault this time since it took them a couple of weeks to release a DRM-free version.
But I agree, it would be best if games were released on GOG the same time as elsewhere.
It makes choosing easier, especially since GOG rarely announces which games are going to be released when.