These are some interesting headcanons about Gaster from this Gaster x FGO fanfiction! Here they are:

1. Gaster communicates solely through Wing Dings using hand symbols, and when he needs to express non-hand symbols, he uses magic to create symbols like raindrops, faces, snow, and geometrical shapes.

2. Despite having the ability to break free from his bond as a Servant, Gaster chooses not to do so because he finds it fascinating to remain in that state.

3. Gaster possesses a Noble Phantasm that has the potential to instantly defeat any target, but the details and capabilities of this Noble Phantasm are encrypted and kept secret.

4. Despite having such a powerful Noble Phantasm, Gaster has never used it and may not even use it in the future.

5. Entry Number ?? represents a series of entries that occur after Entry Number 17, which are distinct and separate from the previous entries.

6. Gaster is aware that he exists within a fanfiction and has knowledge of the Typemoon universe.

7. Gaster is present in all the timelines, but his "physical" form, which is not truly physical, exists solely within this timeline.

8. The Counter Force granted Gaster his "physical" form, enabling him to interact with the world in a tangible way.

9. Gaster knows that Emiya is an original character created by Typemoon and not based on any specific legend or myth.

10. Gaster is aware that I, the writer, am creating this list and is knowledgeable about my (In this Fanfiction) actions.

11. Gaster has successfully reached the Root, but he considers it to be a waste of time since he already possesses complete knowledge.

12. Due to his experiences of being lost in space-time, Gaster could potentially be either the oldest or youngest Servant ever, depending on how time is perceived.

13. Gaster finds alternate timelines where he is involved in "questionable" situations irritating, and this aspect showcases a noticeable change in his personality.

14. Gaster possesses the ability to break Command Seals, the marks of authority that bind Servants to their Masters.

15. Gaster's dorm room's metal door has been replaced with a gray wooden door, which is a reference to Undertale.

16. Gaster is aware that I am tired of creating this list and am without any ideas, demonstrating his knowledge of my state of mind (In this Fanfiction).

17. Gaster finds the individual referred to as "Y/N" interesting and understands that it is a placeholder for "Your Name."

18. In another timeline, Gaster tormented Fatal_Error, and was known as "G!Flowey" during that period. When asked about it, Gaster responds with the hand symbol sequence: ✌️👇✌️👇✌️👇✌️👇✌️👇✌️👇✌️👇✌️👇✌️👇✌️👇✌️👇✌️👇✌️👇✌️👇.

19. Gaster can communicate and be understood by Nursery Rhymes, another character within the fanfiction universe.

20. Gaster has an unexplained aversion to lemons, and it has nothing to do with the fruit itself. The reasons behind this dislike remain a mystery, and even Nursery Rhymes, who knows the answer, refuses to divulge any further information.

21. Gaster's existence in the Nasuverse is highly enigmatic and contradictory. Unlike other Servants who may have multiple versions or variations, Gaster's presence is both ubiquitous and non-existent across the different universes within the Nasuverse. This unique aspect of his existence often leads to speculation and conflicting theories among the characters.

22. Gaster possesses a peculiar sense of humor and often shares his jokes exclusively with Nursery Rhymes. These jokes are delivered in Wing Dings, a mysterious font associated with Gaster, which adds an extra layer of complexity and amusement to their interactions.

23. Gaster occasionally refers to someone named Core during conversations with Nursery Rhymes. Core remains an elusive figure, and the true nature of their relationship and significance in Gaster's life remains unknown. Nursery Rhymes is aware of Core's existence but provides limited information, leaving the readers and other characters curious about this mysterious individual.

24. Gaster has engaged in rap battles on multiple occasions against unknown characters, as revealed by Nursery Rhymes.

25. In a metaphysical sense, Gaster is believed to have created "Deltarune", although Toby Fox is the actual creator in the real world.

26. Gaster perceives Ritsuka Fujimaru (or any protagonist in the Fate series) as individuals with the power to alter fate, viewing them with great interest and intrigue.

27. If Gaster were to exist as a canonical Servant in Fate/Grand Order, he would communicate directly with the player rather than the Master.