I too would love to see the series released on gog.com. I'm not really a fan of Steam in terms of DRM (what happens to the games you paid for when Steam someday shuts down?). Hope a release their happens! Especially for Rebellion since I didn't bought that because of Steam. Would be an instant buy for me on GOG.
Steam has promised they will unlock all Steam DRM if that does happen, but honestly we probably won't have to worry about that for another decade at least. I was skeptical at first as well, but as Steam has only made their restrictions less severe (an actual offline mode for example) I think they may mean it.
As I said before though, it is possible Sins Trinity could be put on GOG, as it has no DRM. However, Rebellion is a steam works game, which means short of Steam actually collapsing, you have to use steam with it.