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.
I have seen more titanosaurs and royal griffins than Gigas. Just why, I saw one, a level 15, disappeared when I was ready to tame it, just why o why o why, they debuff these things and they are as rare as a as all creatures, either make em powerful or make them more common, hit that like button. Let’s hope I find one, again.