smol-goop:

A compilation of all Deltarune discoveries so far related to W. D. Gaster.

• The song that plays during the intro sequence (choosing your vessel) is titled “anotherhim.ogg” as uploaded here. The only other “him” we know of is “mus_st_him.ogg” also known as Gaster’s Theme.
• The version number of Deltarune we are given is 6.6.6. Sixes are heavily associated with Gaster and his followers.
• The same sprite as the Mystery Man’s grey door is used as your method of teleporting in the game.
• If you try to use your phone, the audio is the same as Entry 17′s background noise.
• The song played during credits is titled Don’t Forget, the same words that appear on the photo found in Sans’s basement once interacting with Clamgirl.
• Naming your vessel “Gaster” resets the game; naming yourself “Gaster” closes the game.
• Extreme emphasis on your actions not mattering, a lot like Gaster’s accomplishments no longer mattering in the original timeline of Undertale.
• The bunker found at the end sequence of the surface is mapped as Map No. 17. Changing your location in the code will teleport you there. Also, the sound played when sped up at the bunker is the same sound from Entry 17, as uploaded here.
• This odd egg discovery, both options uploaded here and here. May or may not be related. You get to this room by chance, sometimes spawning there instead of the room with the bird enemy deactivated by a bell. The audio played here is called “man.ogg.”
• If you choose not to continue at the game over screen, the text reads “THEN THE WORLD WAS COVERED IN DARKNESS.” The following song, audio_darkness.ogg, begins with the first four notes of Gaster’s Theme.
• After Jevil is defeated, talking to Seam reveals new dialog speaking of a strange man who convinced Jevil they are all in a game. And what phrase is used? “Darker, yet darker.”
• As discovered here, the process in the beginning to create your vessel is made by a “GONERMAKER” device, hinting to the gonerkid and clamgirl goner being vessels.
• Delatrune’s website is archived all the way back to July of 2016. It consisted of one black image titled “Him.png” with the text "THIS NEXT EXPERIMENT SEEMS VERY VERY INTERESTING” written in Wingdings. On December of 2016, this image updates to read "THREE HEROES APPEARED TO BANISH THE ANGELS HEAVEN" written in Wingdings. Discovery of this info here.

Feel free to add or correct discoveries as we go. I will update occasionally.