This isn't "Gore for McCoy".
This is "Gore AND a starting linebacker AND enough cap space still left to help chase another defensive starter"...for McCoy.
I don't know enough about run-blocking schemes to really evaluate, but it's said that Chip thinks that Gore's style fits the system better than McCoy's did. NOT that Gore is better, just that his relative value in this system is more than you would think. If that's true, it's icing on the cake. Any time you can trade running backs and pick up multiple defensive starters in the process....do it.