1. Taming. I suggest getting a flying mount with good stamina (argy) and using metal buildings to trap it. Then just shoot it
2. Tamed. When the Giga is tamed it loses around 65k hp. And loses a lot of attack. The giga has a bad turn radius thanks to its massive size but makes up for it in brute strength and power.
3. Rage. When a tamed giga loses a lot of hp in little time it will turn into a rage mode which is noticeable by glowing eyes. When in rage mode it will kick you off the saddle if your on it and kill you and your tames.
WHEN TAMED! On a pvp server mostly make sure 100% of the time he/she is in a separate cage completely away from all other Dino's!! This is because after a certain amount of dmg done to the giga he will begin to "rage" he will turn on you even after all the happy memories you've had. Also even attacking all his Dino friends! :( so be sure to lock up this beast of a Dino!
Beware Gigas get nerfed once tamed so if tame try taming a lvl 140+ or whatever is max lvl wild spawn in your server a 20 giga have around 80 k health in wild and will one shot your dinos but once tamed will be reduced to 17k health and superless attributes like 400 stamina which is total shit
I was on my pteranodon which had 300 health and i saw a giga in the distance and i was flying low, i turned around and suddenly with lightspeed the giga comes and gnashes my pteranodon and kills me aswell. Never turn your back to a giga