My top 10 versions of Gaster's Theme. 
Note that despite Soundcloud's numbering system, the #10 pick is actually the one below the original, and the #1 pick is at the bottom. That way it functions like a countdown.

Some rules apply as always:
1. The song has to be for an AU.
2. Covers and takes are allowed, remixes are not.
3. Only one entry per AU. For example, a song and a cover of that same song or alternate version for the same AU could not both be there.
4. Any variations of "!Underswap" (or !Underfell or whatever) all count as the base AU (i.e. Underswap). (Something like Integrity or Undertale: Swapped, however, could coexist on the list)

Slick's Top 10 AU Gaster's Themes

The Monster aka Lemon Demon is Gaster. Not much else to say.

Yep, it's Friday Night Funkin x UT. Had to happen eventually, right? Might as well be us. Some people might not want us making this or think it's stupid that we're doing it, because the game isn't finished and things could change at any moment. That's understandable, but honestly we're just having a fun time doing it, and things will probably change over time to accommodate with the game updating. Either way, it's a nice way to pay homage to it, because as many of you know, it's super fun. Play it here: www.newgrounds.com/portal/view/770371

AU logo by @creepabotinc

"Monster" by bassetfilms
Friday Night Funkin' by ninjamuffin99
UNDERTALE by Toby Fox

Forever Friday OST playlist: https://soundcloud.com/toadiedoesalltheworktbh/sets/forever-friday-ost
Be the first to hear about new updates and other AUs here: https://discord.gg/qmtQxwYKfD

Revealed roles so far (at the time of upload):
Frisk/Chara - Boyfriend
Toriel - Daddy Dearest
Asgore - Mommy Mearest
Sans - Pico
Papyrus - Skid and Pump
Mettaton - Senpai
Muffet - Sgt. John Captain (Tankman)
Burgerpants - Mall Santa
River Person - Limo Driver
Gaster - The Monster

[Forever Friday] The Monster

To top of the holiday treats is this, the "Gaster's Theme" track...sort of! That's what it started out as, but evolved into whatever this is. I don't really know how to describe it, but here it is anyway :D

But the cherry on top of the top is this: FLPs and MIDIs! (a starter selection at least, it'll expand later on with a few more) 

***EDIT 2025: new MEGA.nz URL*** https://mega.nz/folder/XdMTDRJL#wT0eRa-Ggb-qNgDAsRqFGg

The FLPs were made in 20.8.4 and the MIDIs should work properly, I tested them in a few things and everything seemed fine. Let me know if there are any issues.

I'm uploading this on Christmas Eve as I'll be doing stuff tomorrow, so Merry Early Christmas, Happy Holidays, and have a Happy New Year!

(Also fun fact, this was slightly based on my Gaster head-canon, which is that the kind of entity he is now is the same being throughout all universes [so like he wouldn't be subject to role-swaps or atmosphere changes cuz he'd literally just be the same guy]. Even universes that have Gaster still alive would have the "Mystery Man". It's like being a literal paradox, which would be a hellish existence, but hey I'd imagine getting shattered across time and space would do that to a guy. In that context this song doesn't make a whole lot of sense, but it's technically just a non-canonical special track sooooooo don'tthinkaboutitokthanks.)

Transcendence

One of the songs that play in the Sound Test room.

Toby Fox - UNDERTALE Soundtrack - Unused: Gaster's Theme

Megalos overrated.

I decided to fuck around with Harmor after watching some tutorials and I ended up making this.

Original art in the cover by @kompletekrysys

EDIT: Here's an invite for the HD server
https://discord.gg/Pe6qrvs

[Undertale AU - Harmonic Dysfunctions] The Unknown (Updated)

[Disneytale] The Mad Doctor

*̡͡ ͞S̷̡̧a͢v̶e̴͡ ̧o̢͟n̢ ͏̢ȩv͢͝͡e͏҉̡r̴͝y̛͠t̶̶h͟ì͏n͜g͜͠ ̸in͢ ͜҉Ć̷h̀͡r̡̀i͏́s̕҉t́m̀͏̕a͞s̨͠ ̸͠i̧͘͜n ̕[͏̷͝R̶E̕͟DA̧͝C͏̨ŢĘ̷D̡҉].̴

[Terrible Choices AU] - The Forgotten Month

Entry Number 17
* I can feel the heat rising
* The fire is everywhere
* It burns me inside and out
* But I know that this is how it must be
* Those who take no risk aren’t the ones who change the world
* But once I’m done
* The whole world will feel my hellish wrath
* But let the record show that I’m not evil
* I only want a better world for my children
* And the only way this world can be better
* Is by starting over.

Composer: Ebro ( @ethan-sandvich-mcnamee )
Corresponding Undertale Track: Gaster's Theme

Exception

I noticed that in the title of Deltarune, there was a variation of Gaster's motif where the melody paused before the notes swept down lower again. I wanted to elaborate on this kind of melody and try to make a character theme this way. I hope you enjoy my little experiment!

Credit goes to the Spriters Resource for the background used:
https://www.spriters-resource.com/pc_computer/deltarunedemo/sheet/110505/

Underswap - Skelenova

*Day 17.
*Nothing eventful to report, but I must record it anyway.
*Memories have to be kept alive, after all.
*Archived in the recesses of our mind.
*There is no worse fate than being forgotten.

Track EX6 of the Third Wave Survivors OST.

I'd say "meet xReamed (@xreamed), the Gaster replacement," but I don't think they're up for meeting anyone right now.
And besides, it's rude to disturb someone who's concentrating.

Song by: Nelchael Recabarren
Motif(s) by: Nelchael and Sonix
Logo by: @saruky

No, this doesn't replace That One Theme, Y'know? There's REDACTED and then there's Gaster. Here they are two separate entities, each serving their own purpose. Be patient. All will be revealed in due time.

Oh wait, I forgot one of the credits, didn't I?

AU by: ...actually, I don't remember.

In the Archives

Track made by @thetunehero-956661993
thumbnail also by @thetunehero-956661993

join the server!
https://discord.gg/37AubMj

{MultiVersal Mistake} Ost: ???

*To whoever is listening...
*I know my time is limited.
*Soon I will, at last, leave this place behind.
*But that does not mean I will be gone.
*For if you have not gathered it by now...
*I am everywhere.

Another reupload to save minutes, now with flavor text and wonky thumbnail. Still no Quinn ref yet unfortunately, since I don't have a photo of him to work from.

For more info on Schreibertale and many other Slick Silver AUs, check out the server: https://discord.gg/qmtQxwYKfD