I have two suggestions, and I'll leave it to you to pick: Gastro Acid and Hidden Power (Ice) / (fire)
Gastro Acid is so useful this generation, with the high dependency on abilities. Tired of Whimsicott getting priority on setup? Gastro Acid. Trapped by a chandelure? Gastro Acid and switch. Mightyena trying to get free speed boosts? Gastro Acid, then Bug Buzz away! There's lots of room to get creative with Gastro Acid, as it has variable potential. It's a really great utility to have on a speedy little devil like Galvantula.
Hidden Power (Ice) / (fire) is mainly there for the coverage against potential counters. Ice hits Salamence, Garchomp, Flygon, and Altaria all pretty hard. It can hit back at Serperior, as Galvantula is considered a potent counter for it, as it blocks off choiced Leaf Storms. The fire-type variant is mostly there for bulky Steel-types like Durant, Escavalier, Ferrothorn, and Registeel. It also melts Ice-types like Abomasnow, Cryogonal, and Vanilluxe. And of course, it can get rid of the few Grass-types that neutralize Bug Buzz, such as Victreebell and Amoongus.
Take your pick, Buddy. This is what I'd recommend!