When encountering a Giganotosaurus in the wild, do not engage, even the toughest of tamed Gigas fall victim to them. If you haven't been spotted, run in the opposite direction. If you have been spotted, run past the nearest herbivore, it will draw its attention so you can get away.
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