Yeah, but it wouldn't actually do much would it...
Like, there is no real bonus from gear stacking when you have haki. It just makes you a more obvious target. Not to mention, Luffy is already faster with gear 3rd now. He wasn't using gear 3rd as a gattling attack pre time skip.
Also, again, Gear 4th is significantly better even in spite of the 30 minute time limit.