Re: 6.5 Creedmoor or 6.5 Grendel
What I found after loading and shooting the Grendel (in a really nice set-up BTW) is that it will get you to 1k or a little farther just fine. It won't do it competitively with the 6.5x55, .260 Rem, and 6.5 Creedmoor. It WILL fit in an AR-15. They won't. They, will fit in an AR-10. Which is spendier, but just as accurate as the AR-15.
With a bolt gun, all of the longer rounds mentioned, will fit in a short action. Although I prefer a long action so I can seat bullets out a ways.
So, my opinion is since you want to go with a GAP build, go with the biggest round you can stick in the action of your choice.