GOG.com have just launched a service called GOG Connect, which allows you to hook up GOG and Steam accounts and redeem certain eligible games on the former. For example, if you own FTL: Advanced Edition on Steam and combine the accounts through GOG Connect, you’ll now own a DRM-free version on GOG as well.
There are twenty-some eligible titles at the moment, with more titles promised in future. All presently eligible games are “limited time offers” made possible through developer and publisher co-operation, but once redeemed they will stay on your GOG account indefinitely. Steam-specific account stuff like cloud saves and achievements won’t transfer, but local save files will (in most cases) work across both versions.
It’s only possible to connect one Steam account to your GOG one, and the process is permanent, so make sure you’re match-making with the right pair of accounts.
Here’s how to do that:
1) Log in to, or create, your GOG.com account.
2) Log in to your Steam account via the secure form.
3) Use the “Import Games” button. You can deselect any games you don’t want to import – otherwise all of your eligible games will be added to your GOG.com library.