Where to Find and Catch Gigalith
From Pokemon Go Wiki
Gigalith are found and captured in Nest and Habitat locations.
Nests
Gigalith Nests are locations that consistently spawn Gigalith.
Habitats
Gigalith Habitats are spawn locations where Gigalith is likely to be encountered, but not guaranteed the way a Nest is.
Additional Tips and Information
Best Nests
If you know a really good nest or any of the nests above is really good, list them here: