The safety and the bolt catch lever are the only things that don't come ambi. @ Dave I would love to see photos of the 60 degree levels on the GAP-10 (in both positions).
As for the caliber, if I was in the market again I would go 7mm-08.
Here is my reason:
Cheap but very nice LC brass can be bought here
Lake City 308 Brass - Processed - Texas Brassworks just neck it down to fit the 7mm
Using the I believe 7mm 162 grain A-Max
Should be good out to just shy of 1500 meters.
The big bonus is you can find the parts easier than the current flavor of the month, 6 CM or 6.5 CM
Might even give the .260 a better look (again using .308 brass)
The barrel: 20-23 inches is up to you. The barrel length is just free velocity. I run a non-fluted 18' cambered in .308 and shoots 1/4 groups with my hand loads with photos for proof. I believe when it comes to precision and more over precision in quick succession, I want a thicker barrel. No barrel will be stiffer if you reduce material and overall diameter. If your shooting out to 1000 yds only you are not offhand shooting meaning weight is now no longer an issue. If anything the added weight of not fluting will reduce felt recoil and make it easier to keep your eyes on target.
Just my 2 cents, and that is about all it is worth.