For those who wish to Mega Evolve your Gengar into Pokémon Go, you will need to acquire enough energy to transform it. Unfortunately, the only way to gain Mega Gengar energy will be by completing specific tasks on the field or battling Mega Gengar in the game.
Mega Gengar Raids will be available from 1st to 22nd October. For those who have battled Mega Gengar before this time, it is a good opportunity to refresh their energy reserves and store them for later. Like the Pokémon Candy you receive for catching and transferring a Pokémon, Mega Energy doesn't contain a place for your inventory, so you don't have to worry about making room for it.
There are also a handful of field research missions that you can receive by rotating Pokéstops. You will need to find these positions in the game and receive them in one of your three Field Research slots. Of the two, Mega Gengar Raid Battles are much easier to grant Mega Gengar Energy, as long as you work together with a party. However, field research tasks are easier to do alone, but the task you get from a Pokéstop is random.
Mega Gengar will be available for a limited time. Mega Absol will be the next Pokémon to appear in mega raids starting October 22.