Nests are areas that a specific specie appears regularly and frequently. A player can easily find and capture a particular wild Pokemon. Tap the link below to see the details of Pokemon nest.
This page notes what is a Pokemon nest, a Pokemon nest location, and a rare Pokemon nest.
Gastly
Here are the list of Gastly nests. Some of them are confirmed by players but others are rumor. Go Radar map screenshots are put to see how often Gastly can be found in each location.