What a nice idea: get the GOG version of a boxed PC game you own by simply entering your old key code there.
The buzzword is "reclaim", and at the moment you can only do it with a handful of games - headlined by the popular S.T.A.L.K.E.R. series. It also means you get the added GOG bonuses each game may or may not have, plus all the compatibility headaches taken away.
S.T.A.L.K.E.R. maker GSC Game World told me today that your old Gamespy codes will also be redeemable on Steam as well. Whether Steam will offer broader reclaim support beyond S.T.A.L.K.E.R. remains to be seen.
Oh and while we're on the subject of S.T.A.L.K.E.R.: there were small updates made to the game recently and Redditors were scratching their heads as to why. Turns out the updates were specifically to resolve an Nvidia graphics card issue, GSC said.
Keep an eye on GOG's dedicated reclaim page for when additional titles are added in the future.