If there was a battle for Gaster the battle theme would definitely contain somewhat of another "skeleton" battle theme because of his connections to Sans and Papyrus. And that's exactly what you have done. You also kept it creepy and sinister just like Gaster himself. Well done.
If Gaster every needed a battle theme I would definitely support this.
Please contact me if you would like to use this in a project. We can discuss the details.