Hi @goodwebsites
The only way to do this is to hook into the save function of the gallery field (before ACF saves the new value) and compare the previous and new values.
From there you can delete the attachment post records you need to.
Use the above search to find more info on the acf/update_value filter which you can use to hook into the gallery field save function.
The rest should be straight forward but will require time and debugging skills.
Good luck
