The giga is one hell of a dino. If your use to taking out Rexs with a mastercraft double barrel shotgun.... do not make the same mistake I did in thinking you can fight it. You simply can not fight that thing. Just run. Just get on an iguanadon or a flyer and run as fast as you can. Also, if you happen to be mining crystals between Griffin Mountain and the volcano. Watch your back because that is where they spawn.
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 spent an hour taming a Max lvl anky on a ledge. After about 20 minutes I looked down and 2 gigas were below looking right at me their heads almost reaching the top of the ledge. They stayed there the entire time