Fever_Discordia: Wait, hold everything, has GOG even defiantly confirmed that they're building GOG Galaxy themselves, from the ground up?
I'm pretty sure, from the update process that both the Steam and Origin clients are re-skinned (and heavily customised) versions of the same basic technology - don't know whether that's Origin basing itself on Steam or if they've both gone to the same 3rd party to build their clients
My point is, though, that we don't even know for sure, at this point, that GOG are going to own all the technology that Galaxy is based on and not be the licensees of someone else and therefore might not even have the rights to open up all or even part of the source!
GOG Galaxy is written in C++ using Chromium embedded to display its GUI. It also uses a number of other open source libraries for things like networking.