Max Starfall inflicts damage and causes the battlefield to become Misty Terrain for 5 turns (8 if a Terrain Extender is held). Its power and whether it is a special or physical move depends on the move it is based on.
Gigantamax Gengar makes multiple yellow stars fall down from the sky and onto the opponent. When the attack hits, a multicolored star-shaped burst of energy is released.
This article is part of Project Moves and Abilities, a Bulbapedia project that aims to write comprehensive articles on two related aspects of the Pokémon games.