Where to get gastly on Pokemon Gold?

You can't find Gastly in Pokemon Gold. Gastly is a Generation I Pokemon, and therefore only appears in the original Red, Blue, and Yellow games.

However, if you're playing Pokemon Gold, you can still encounter the evolved forms of Gastly:

* Haunter: Can be found in the Union Cave on Route 12, where you can also find the Ghost-type Pokemon Gengar.

* Gengar: Can be found in the Union Cave on Route 12, with Haunter.

If you're really set on getting Gastly, you could consider trading with someone who has a copy of Pokemon Red, Blue, or Yellow.