So the real question is when the AAA strat is down, why do GNG guns at a Vehicle base only stay down for 15 mins & BAS guns go down for longer times?
You got it the wrong way. GNG (and all other auto guns you find on ships, for example) stay down for 30+(strat bonus) minutes. Downtime can also be shortened by resupply.
BAS are manned guns and stay down for 15 minutes fixed (just like all other manned guns, for example shore batteries). They can't be resupplied at all.
The 15 minutes for manned guns is equivalent to the hangar downtimes of 15 minutes (both items 'spawn' players).