My guess as to the reason why no one does this is because it doesn't really make sense to. If you have to convert your whole engine and engine management to another engine, why would you go with a slightly stronger 4 cylinder? Most 4 cylinders, even modern, are still not as powerful as the Jeep 4.0 and certainly not a V8, unless they have a turbo slapped on them. Assuming the GM engine you are talking about is the Ecotec 2.5L, which is 190hp and 171 ft lbs of torque. Haven't seen the dyno on that torque curve but my guess is you have to rev it way high to get any of that power seeing as it's a DOHC engine. And 171 ft lbs is not really all that much power, certainly not worth doing a custom setup for, especially when the rest of the world hasn't done it so you would be the guy figuring everything out.
On the other hand, you can get an immediate huge power increase with the 4.0 (roughly 220 ft lbs of torque and it's down low) and it's really not that hard to swap to especially since it was an OEM setup. Collect all the parts, weld on new frame mounts, and you're basically good to go. If you're going custom swapped engine, then you may as well go with a V8 to maximize your power return after you invested so much into it.
I have seen a few people "build" the Jeep 2.5, I have never seen any good results come from it. I've pretty much only seen eBay turbos or the 505 performance stroker. Neither with great reviews and based on their accel videos, I am certain my regeared 4.0 would still smoke them.
If you don't need a ton of power, then just regear the 2.5 appropriately and drive it as is. If you want more power beyond that, swap the 4.0 if you want something easy, or reward yourself with a V8. Don't bother with a bunch of time on another 4cyl solution. My thoughts, anyways.
Not trying to be a downer, but going with a different 4 cylinder makes no sense to me when I look at the logistics of it. Better off keeping what you have and gearing it appropriately, or swap to one of the more popular, much more powerful options, IMO.