Why would you go back to Steam? If a game is available on both GOG and Steam you always buy on GOG.
If I buy something on GOG I basically "own" the game. Assuming I download the offline installers and take care of my personal backups (you're stupid if you're not backing up your offline-installers) no one can take that away from me.
That's not the case with a rental service like Steam. Valve has straight up said that Steam is a rental service. If something happened to Steam or Valve, well, you're entire library is more or less gone. If something happens to GOG, well, my games are fully useable and in my possession. I'm not dependant on some shitty App like Steam to install and retrieve the software I paid for.
GOG = Here's your purchased software. It's your's to keep forever, and even if something happens to us. Just make sure you don't lose it!
Steam = Here's access to your "lifetime" rental. If something happens to us though, you'll no longer have access to our service and therefor any of the software you thought you "owned" when buying from us.
But seriously, GOG is not going anywhere. The Summer sale will be starting soon (hopefully tomorrow) and you should always buy with confidence. Feel good knowing that if you buy something on GOG you're getting the best version of that game possible, because it's one that you have control over and actually "own".
No GOG = No buy.