The GP-6 is the old model and is not available in the most recent Snell ratings except in size XL. They didn’t make an XL shell for the updated GP-7. The XL GP-6 is still approved for competition but all other GP-6s are rated SA-2015 or lower.
The GP-7 has a little different interior padding configuration but generally fits the same. The cheekpads cup under your chin a little better for a more secure fit. The GP-7 also has more ventilation and a different visor and different hardware, as the eyeport is redesigned for more forehead protection.
For context, I am an Arai dealer and painter who has disassembled and reassembled hundreds of 6s and 7s, and have worn the GP-6 and am currently finishing up my GP-7 for this season.
The GP-6 and 7 are two different models. The GP-6 is the old model. But it is only available new in size XL. If you are seeing GP-6s in any other size, it is just old stock. There may still be some SA-2020 versions of the GP-6 floating around, I’m not 100% sure. I know Arai America doesn’t have any GP-6s left in the warehouse. The GP-7 replaces it.
If you can find an SA-2020 GP-6 and it’s cheaper than the 7, I’d go for that. The difference between the two is minimal. Otherwise if you want the newest and nicest, the 7 is your go-to.