A “descending gandhabba” just means a gandhabba entering a womb. I am not sure why it is stated as “descending.”
You wrote: ” So maybe the Buddha copied this idea of the Brahmins.”
That means you have no idea who a Buddha is. A Buddha does not need to “copy” anything from anyone.
You can believe or not believe anything. That is totally up to you. But refrain from making derogatory comments. Just ask a (sensible) question if you like.