I've been playing with this concept. Very interesting. The idea that enemy forces are a limited resource that you can use to generate a variable amount of GG points, depending on how you use them. I like it.
As with many things in the game, there are pros and cons. Also, like other things, you can spend a different resource (production, in terms of dead units) to increase this resource.
One thing that doesn't quite "fit" in this puzzle, though, is the way the Great Wall works. It doesn't allow Barbs in your borders. However, it gives double GG points for combats in your borders. So, the "trick" works for AI but not Barbs. Well, I guess you can't have everything.
Wodan
You are right

Also what I was advocating by attacking at lower odds (to increase exp gained) was indeed a way of trading hammers for GG points. I would point out that the GG points can also be converted back to hammers if used to build a military academy, where the 50% production bonus for military units effectively generates extra hammers. A city with a modest 20 base hammers gets a 10 hammer bonus each turn that is used to build units which is a very strong effect compared to other GPs. I can see both of these effects happening but I can't yet determine how far we should go in terms of attacking at low odds to generate GG points. As mentioned in my original post the situation is complicated by the effects of WW.