I just did this. I got over 1000 games on steam, but only 13 transferred over to GOG. I figured there would be more. Oh well. At least I got somethingGOG.com
If you link your Steam account to your GOG account, GOG will give you a copy of select games so you can have a DRM-Free copy without having to buy it twice. Games are only available for a limited amount of time.