Transfer Steam Purchases To GOG With GOG Connect
https://www.gog.com/news/introducing_gog_connect
This offer allows you to add games you own on Steam to your GOG inventory. It is available for specific titles only and the offers might only appear for a limited time. Still, it'll allow you to get extra digital copies for free.
The current list of games on the page linked above is not quite complete. I could transfer Braid over to my GOG acct, but I don't see it listed on the page. You'll get the list of current offers if you link your Steam acct to the GOG one and use the actual service. Some of them said they're valid for five more days when I did that just a while ago.
This is just a heads up for people who use both and don't follow GOG news regularly. The games that are offered this way depend entirely on the publishers allowing it, I believe.