I saw a giga and a tyna (idk why the tyna isn't on dododex) on the green obelisk mountain just go to the top edge of the green obelisk island. but it might not have one. Also there's a explorer note on the mountain
I killed a level 150 giga on the center with 5 rexes that all had about 40k health and between 700-1400% melee damage and 100% imprint. Only the rex I was riding had a saddle. Nobody died, and they each had about half of their health left.
You can kill it, you just need to be very prepared.