I used to build for other people then test fly and trim the planes before I handed them over to the new owners. I have never owned the GP Extra but flown several, I have owned and built several of the CG Extras.
The CG Extra comes out more on the {what is thought today} heavy side compared to the GP Extra. The added weight seems to make them much smoother and track better. They fly about the same other then that. If you want to compete using one of these planes the CG is the better choice. Any weekend sport pilot wouldn't even care about the difference.
The CG is sold as a 60 size plane but a 60 two stroke really doesn't allow it to come alive. The ones I have built have all used a 1.20 four stroke, both OS and YS and up to a YS 1.40. The one with the 1.40 was the best of the bunch. I'm sure either one of these two kits would handle the little 20cc gasser very well.
I'm sure both kits would be OK with the little Zenoah in them. I once owned an ARF {I know!!} Extra 300 with a 73 inch wing span and I was using an RCS 1.40 {22cc} gasser in it. The plane flew IMAC very well but could have used more power, it came in at 13 pounds.