The many ways to kill a wild giga:

Bite its tail with literally any flyer (Tapejara griffin or tropeo recommended)

Shoot it with a rifle from the back of a griffin/tapejara

Use a lightning/poison wyvern

Spit at it with a basilisk

Shoot it with a velonasaur on the back of a quetzal

Sic a titan on it

Lure it into deep water and use any aquatic dino

Freeze it with an ARK: Additions Cryolophosaurus and retreat when it unfreezes

Use hit-and-run tactics to bleed it to death with a speed leveled carno

Shoot it with a tek saddle (preferably tapejara or rock drake)

Blow it to hell with a saddled Tropeognathus

Press tab and type 'kill' while looking directly at the giga

More Giganotosaurus Encountering Tips