If you're on a land mount and the Giga has you in its sights... you are completely helpless. If you are on a Air mount, stay a good distance above it, as its hitbox is massive, also don't forget to watch your stamina!
If one is encountered near water, you can simply lure it out to sea with a flyer and it will swiftly drown. Given the water is deep enough. If encountered inland not near water. Pray to the overseers you have something fast enough to out run it. Then promptly die because they're as fast as a mad Hispanic mother with a shoe. Not as deadly as one though.