Although he does not appear physically in the game's main story, he is alluded to through cryptic references in rare dialogue from minor characters.

He is also indirectly referenced during a fight against Sans, who wields weapons that are labelled "Gaster Blasters" in the game's files.

[4][better source needed] According to IGN, the "Mystery Man" sprite appears to be based on the design of the character Uboa from Yume Nikki.

[2] Players can also modify the game to place their character in an unused room internally labelled "room_gaster", containing a message written in the Wingdings font titled "ENTRY NUMBER SEVENTEEN", in reference to a missing entry in a series of logs in Undertale's main story.

[7] This update added a rare event that would allow the player to enter a room containing the "Mystery Man" sprite, which was originally only possible by modifying the game's code.

[12] A rare possible event in the town of Snowdin in Undertale has the protagonist receive a mysterious phone call asking "Can I speak to G..." before the caller decides it was a wrong number and disconnects.

There is no way in the Japanese syllabary to represent only the letter "G", and an early draft translation by 8-4 used が (Ga), which would have been taken as further confirmation the line referred to Gaster.

[3][18] Kotaku writer Nathan Grayson noted that W. D. Gaster was the only secret left remaining in Undertale, discussing how, despite all of the work put into investigating Gaster, the mystery still remains unsolved, suggesting that the data in the game files could have been leftover content not intended to be found.

[2] Tunic game designer Andrew Shouldice spoke of his appreciation for "risky secrets" like Gaster that cause players to "consider a new approach that'd never crossed their mind, or reveal a submerged mechanic that they'd never interfaced with.

[6] Kotaku writer Heather Alexandra suggested this could be the case, citing design notes that allude to a character related to Papyrus and claiming that this could be Gaster.

[2] In reference to the NPCs found through file modification, some believe that they are fragments of Gaster speaking about the experience of existing across time and space.

[2] Writer Mattia Podini discussed the presence of techniques he suggested were used by Toby Fox to manipulate players into responding to certain details in a certain way.

For Gaster, he suggested that he served as an example of players experiencing apophenia, the perception of meaningful connections between unrelated things.