Chapter 56 - 56: The Mysterious Stone Pillar
A Pokémon's innate quality varies, and this translates into different talents. While it may not be evident at lower levels, as they level up, these differences become clearer.
A Pokémon with high innate quality is superior in all aspects: learning, battling, and synchronization with its trainer, compared to a Pokémon of lower quality.
In essence, a Pokémon's innate quality almost always determines its maximum potential.
Returning to the main point, if this Gengar was placed here, then what Gastly said makes sense. Since it was stationed in this spot, it has been absorbing the Ghost-type energy that inhabits the area.
This was likely the main reason Umbra left Gengar here.
Gastly mentioned that this cave was originally brimming with Ghost-type energy, enough to sustain the entire colony. Even some Gastly with good innate quality could evolve into Haunter.
However, since Gengar's arrival, the energy in the cave began to decrease drastically. So much so that even those who had already evolved into Haunter reverted back to their former Gastly form, which is extremely harmful to a Pokémon.
Gengar, being the final evolutionary stage of Gastly and Haunter, naturally exerts great pressure on them. And moreover, this isn't just any Gengar; with such high innate quality, the Gastly—most of whom are of red quality—stand no chance of resisting its influence.
To avoid being absorbed by Gengar, the Gastly were forced to move to the outer areas of the cave. Even though the energy is much scarcer there, it's still something.