My personal take on Gaster's battle theme
(This being a personal take doesn't mean I believe this wholly to be true or it being a theory with evidences supporting it. This is made for fun. You can post your own takes as well!)
(BONUS: My own take on the battle scenario. Gaster is depicted as Redacted here because it is his final form, just like Spamton in his NEO form).
Gaster's battle theme would be an arrangement of songs such as Freedom, ANOTHER HIM, Darkness Falls, and Don't Forget.
CORE / Another Medium / Patient could also be included. An arrangement of 'D.ogg' + Flashback and The ID from Earthbound: Halloween Hack could be used as the intro and outro respectively. The Legend + The Dark Truth could appear in certain segments, moreso if said segments are intense.
If Gaster in Deltarune is the same from Undertale, Once Upon a Time could be expected. Mus_smile (i.e. the garbage noise) and Giygas' Static sounds from Earthbound could appear occasionally at the background, emphasizing Gaster as a being beyond our comprension. Also, an arrangement of Penumbra Phantasm would be interesting to hear in different parts of the theme.
General (Title, Length, Genre):
As for the title, options like "AGAINST HIM", something referencing the devil, such as "BATTLE AGAINST THE DEVIL", or maybe a parallel to Don't Forget, something like "REMEMBER ALWAYS", could be suitable. If Gaster as a character is a reimagining of Halloween Hack's Dr. Andonuts, the title could be a nod to Dr. Andonuts' Rage (the original file name for Megalovania), for example, "WRATH OF THE BROKEN DOCTOR."
Alternatively, the title could be related to darkness, shadows, or Gaster's entry number seventeen as a whole.
The duration of the track would be around 6 minutes, as Gaster is related to the number six, making it Toby Fox's longest track in Deltarune (as of now if he will keep making music ranging up to 2-3 minutes in length. On a side-note relating to this, the full version of Penumbra Phantasm could be one minute longer but who knows?).
As for the genre, it could be dubstep, heavy metal, or even experimental, a play to the nature of Gaster's character as the former Royal Scientist.
That should be all from me.
Tl;dr. Gaster's battle theme is somewhat full of motifs and is very long in length.