Where to Find and Catch Grumpig
From Pokemon Go Wiki
Grumpig are found and captured in Nest and Habitat locations.
Nests
Grumpig Nests are locations that consistently spawn Grumpig.
Habitats
Grumpig Habitats are spawn locations where Grumpig 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: