GOG has introduced something called "GOG Connect", which basically allows users to have a free GOG copy of games if they are in your Steam account. You do not lose access to your Steam games...nothing happens as far as Steam is concerned. You just get the games added to your GOG account.
You have to link your Steam account to your GOG account, and this is permanent. Basically, this just means that once you link your GOG account to your Steam account, you cannot link your GOG account to a different Steam account. Nor can that Steam account ever link to another GOG account. It is 1 account to 1 account.
Currently GOG Connect only has 23 eligible games, and GOG Connect is only available for a limited time on these games. This promotion started on June 1st, and I believe it lasts 5 days, so time is running out. Do this tonight or tomorrow, or possibly lose the ability to do so for these specific games.
GOG has stated that they will make more games available for this in the future. Personally, I am going to check back on the 6th and see if they added any more. Or maybe they will just do it once a month or something...who knows.
Last bit of relevant info is that you have to make your Steam privacy settings public for GOG to check what games you have and for the linking process to work. You can change it back to whatever settings you had it at before after you have finished linking the accounts and getting the games credited to your GOG account. I assume if you ever do the process again later, you will have to change your Steam privacy settings back to public until the process is again complete.
Anyway, I should have posted this earlier, but I forgot about it. Better late than never! ;-) But time is running out on these initial eligible games, so again, get on it if you are going to take advantage.
Credit to Gruftlord on the AFKMods forum for initially letting me know about this. Link.