Gigas is in Praenost, in an area known as Mount Garrison. To get there, you'll first need to get to Pioneer's Plain (which is the area where the Preaneost Castle is).

In Death Stranding, some pretty crazy terms and words like DOOMS, Chiral Allergies, and Aphenphosmphobia are thrown your way early on in the story, and many of them never really get explained.