So GOG.com has just rolled out it's GOG Connect program in which you can permanently join your Steam and GOG accounts. The idea being that certain participating games will then show on both accounts. The list of participating games is fairly small but we are promised it will get bigger as time goes on.
Also I want to point out that this joining of accounts is permanent and irreversible so don't try and link your GOG account with a secondary Steam account thinking you can sever the link later.
This should be a nice way to take advantage of GOG's DRM free setup files and extras for games you may already have on Steam. For me, I'm particularly happy about Unreal Gold and Unreal Tournament participating.
Best of luck and enjoy your games!