A human gandhabba will be born with a human body many times during that human bhava.
An animal gandhabba will be born with an animal body many times during that animal bhava. For example, a “dog gandhabba” will be born only as a dog, not as any other animal.
The gandhabba state remains that same until that existence (bhava) runs of kammic energy. When that first dog dies, the “dog gandhabba” comes out of that dead body and waits until another suitable “dog womb” becomes available, at which time it goes into that womb and will be born again as a dog. That dog may not look the same as the first one, since the parents are different. But it si the same gandhabba.
Gandhabba is the “mental body”. It cannot change from a human to a dog or dog to a human, until the kammic energy for that bhava runs out.
But if a human dies the last death in that human bhava, he may be born as a dog in the next bhava. OR he can be born as a deva in the next bhava, in which case there is no gandhabba. A deva will be born with a full deva body and live in that way until the kammic energy for the deva bhava runs out.
See, “Bhava and Jati – States of Existence and Births Therein” and “Gandhabba – Only in Human and Animal Realms“.