What Weclock said. Also this is a crafty way for gog to drive some sales and quite a good one too.

So wait, lemme get this straight...
I own Simon the Sorceror 1 & 2 & I don't *need* to re-buy them as I just use the originals with SCUMMVM.
If I wanted to participate in this competition, I'd have to spend $5.99 to potentially get $5.99 off another game? Would it not just be prudent for me to buy the other game i'd get as 'free'?
I understand why the rule is there (to drive sales) but seems a little pointless. Surely just the ideal thing to do should be offering 1 free code to one lucky reviewer and should be open to everyone who has a valid opinion on the game?
The way its currently working is more like a lottery and less like a competition.

I feel like the way it is right now is sensible enough. It drives sales, and you make sure that whoever is reviewing the game has played it and is not just regurgitating what he has read somewhere else.