The term gandhabba is normally reserved for the “mental body” (kammaja kaya) of a human or an animal. In most other realms, the living being is formed with “a full body” at the cuti-patisandhi moment.
The preta realm belongs to the apayas, i.e., it is one of the lowest four realms. Animal realm also belongs to the apayas.
So, there is a huge difference between a human gandhabba and a preta. There is also a huge difference between a human gandhabba, animal gandhabba. The word preta is not used to refer to a gandhabba.
I think the possible confusion may come from the fact that some pretas have very fine bodies, just like (human or animal) gandhabbas. But brahmas also have such fine bodies, and brahmas are very different from gandhabbas or pretas.
The following post provides some references from the Tipitaka for a human gandhabba:”Gandhabba State – Evidence from Tipiṭaka“.
Without the concept of a gandhabba, it is not possible to have a consistent picture of human and animal realms. For example, the difference between bhava and jati is impossible.
These are discussed in the following sections. You can scan through and read those of interest:
“Mental Body – Gandhabba“.
When the gandhabba is not inside a physical body (when it is waiting for a womb), it is in “paralowa“. Not believing in paralowa is one of the 10 types of micca ditthi; see #5 of “Ten Immoral Actions (Dasa Akusala)“.
P.S. Any sutta that discusses the 10 types of micca ditthi will have the reference to “para loka“. Actually, the term “parolowa” is the Sinhala word for the Pali term “para loka“. I need to revise those posts with the word “paralowa” with “para loka“.