Hey, guys, lay off. The OP is NOT asking about the merits of one over the other. He HAS posted he looked. It's a choice with some thought and balance in what he wants.
As for actual ballistic application, either round sighted in for the distance will do, and can hit the same candy wrapper. The real issue is does the 6.5G offer him any extra ftlbs at the longer range he can use. The answer is if he's willing to use the longer barrel to get more initial speed - the shorter one won't deliver as much. It doesn't mean it won't at all.
Other than the bolt, barrel, and magazine, everything else about a 16-18" AR for hunting is the same as for the other caliber. The specific parts listed look like good choices. Barrels are available from AA, Les Baer, and others, along with complete uppers. Check in with the apparent 6.8SPC makers too as many offer 6.5G barrels. It's business, they sell what they can.
Shoot equal rounds - bullet weight and powder - down equal length barrels, and there isn't that much difference. The actual performance will be apparent, 6.8 is generally faster with more power at short range, the 6.5G is generally slower but carries more power at longer range. That has to do with a bigger powder load and shorter bullet vs. less powder but more aerodynamic, that doesn't lose velocity at longer ranges.
The general consensus I've gathered over the last year is that the G will do slightly better from about 250 to 400 and beyond. Either will do about 40% better than 5.56, the whole point of the effort. Whether the slight difference is worth the commitment is a different issue, both calibers have been misunderstood and sold off. Apparently some think they should be able to treat them like 5.56 and demand .25c a round ammo at every possible shopping location. To say their expectations are unrealistic understates the problem. Those flash in the pan fanboys aren't the backbone of the user groups by a long shot.
WHO HAS actually used a 16-18" Grendel and what are YOUR impressions?