alright, so this is an absolute shot in the dark, but...
FICFINDERS! I REQUIRE YOUR SERVICES!
heres what i do and dont remember; RED is for unknown/excluded, ORANGE is for unsure/may not have been tagged, and GREEN is I Am Reasonably Sure That This Is Correct But My Memory Is Shit.
Platform: AO3
Rating: UNKNOWN
Archive Warning: UNKNOWN
Category: UNKNOWN
Fandom: Undertale
Relationships: W. D. Gaster/Grillby, W. D. Gaster & Asgore Dreemurr
Characters: W. D. Gaster, Grillby, Asgore Dreemurr
Additional Tags: Royal Scientist W. D. Gaster, Pre-accident W. D. Gaster
Author: UNKNOWN
Date Posted: Before or around DELTARUNE ch. 1 (~pre-October 2018)
ADDITIONAL INFO:
i remember there being a scene in which Gaster was talking to Asgore about... something? and Asgore mentioned purple (Gaster's magic/soul color) and gold (Grillby's magic/soul color) as being a good color combination, obviously in reference to Gaster's crush(??? Grillster could have already been dating, unsure). the colors of grillsters souls were brought up a lot.
i THINK that this fic did not have Toriel, Sans, Papyrus, or Frisk. the fic was set after Chara and Asriel died, and Toriel had left for the Ruins. they may have been mentioned, but did not appear in the fic. (unless i am wrong, which, very possible. this was a couple years back.)