Gravity is a force that is inversely proportional to the square of the distance between the two objects.
G ∝ (Mm)/ r²
As Cosmoem is very, very close to Earth (or PokeEarth) its square becomes very small indeed (Remember this is in comparison to planet size). Further, Cosmoem is only 999.9kg. I say only because the planet weighs a LOT more.
Hence, it can be assumed that Cosmoem is a part of the isolated system, or part of the planet itself.
Maybe, if Cosmoem was bigger, heaver (by thousands or hundreds of thousands of times), and were sufficiently separate, you could observe the effect of Gravity on each other.
In fact, the basic principles of classical physics are such that every object attracts the other. Only, in case of objects on Earth (or PokeEarth), they are just so small that their insignificant mass tremendously reduces the force between them. In a way, you and I also attract the planet, but our gravity is so, so small in comparison that it can be neglected.