A Gargantuan Yeti is a side quest in the Orient (Act III). It is given by Pasang Do-Ma in Shangshung Village. Talk to him to learn about a giant Yeti dwelling in the ice caverns beyond the Chumbi Valley.
You will hang onto this quest for some time, it is completed a couple of regions later. The Gargantuan Yeti is found in the Tsongmo Ice Caverns, which lie to the east of the Chumbi Valley (you won't miss him, his lair lies on the main route).
The Gargantuan Yeti is a moderately strong and fast melee attacker, but moves very slow. He uses a Freezing breath and an ice projetile attack dealing Cold Damage. However, he's usually not too difficult to defeat.
Reward: 20000 experience