Should your Gengar know focus blast or dream eater if it knows hypnosis?

Gengar should know focus blast as Dream eater is a bad move when Gengar is not running any sleep supports (ie: Will o Wisp + Curse), as it prevents gengar from switching out if the target is swapped out. In a situation where Gengar is running Will-O-Wisp + Hypnosis, then dream eater is a good choice as it doesn't take damage when hitting a burnt foe, and prevents the opponent from switching out.

In short:

Focus blast: Generally better utility and damage

Dream Eater: Better in specific scenarios