The GP division is mostly 5.56 rifles, interspersed with 6 ARC, 6MAX, and shorty .308 and 6.5 Creed. Very low number of anything else, and I'd say 75% 5.56 with everything else combining to make up the remaining 25%.
A 4-16x optic will put you into Practical Precision even with a 16" barrel, and that division is a pretty even distribution of calibers, favoring large frame a little.
Frankly, I feel like Practical is a more "new to the game" friendly division than GP, as you have enough glass to see targets, impacts, and misses. A lot of newer GP 5.56 shooters get frustrated because they can't see where the misses are. If you go .308 you also get a bit of a benefit with the additional energy on target, making spotting for you AND the person on glass easier. If you can manage 308 recoil, of course.