It'll be interesting to see how this works out. Gaming is definitely a luxury, so it is something that can be cut. Good games will still sell though, bad games will drop fast (but, then again, they seem to drop pretty quick now).
You may be right on the buy 2 get 1 though. There are strong margins in the used market, so if Gamestop wants to move product, they could run such sales. I have a feeling they will have some strong sales during the holidays, whether that be B2G1 or the % off coupons.
I disagree with the poster that people in a bad economy would be more likely to demand more for trades. My guess is that people are more desperate during these times (either needing money for their trades or the only way to get a new game is to trade in older ones). So, if anything, I could see Gamestop trade in values get worse during this time, not better.