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1.Gerson, Asgore and gaster used to be best friends as kids and later adults
2.They loved pretending to be the Delta Warriors from the prophecy as kids
3.Gaster was the nerdy & shy one, Gerson was the Rough one with a big heart & Asgore was the leader and always ready for adventure and was the one who always gave advice and hugs
4.During the war they protected each other as much as they could
5.Gerson & Gaster supported Asgore after Asriel & Chara’s death
6.Gaster was Asgore’s royal Advisor before he was the royal scientist
7.Gerson and gaster were ‘uncles’ to Chara and Asriel before their deaths
8.Gaster was the glue that held the friendship together, after he was shattered across time & space Gerson and Asgore rarely saw each other
9.They were all Teens/young adults during the war, Gaster was a field medic
10.Gerson lost his eye trying to protect human children, they unfortunately died, & when Gaster tried to heal him to get it back Gerson didn't want his eye back , as a reminder
11.After the war Gaster despised humans for almost wiping out the skeleton race(I'm taking some headcanons from hand plates) , he was one of the only skeletons left
12.They met up every week for tea to talk
13.Gaster had some sort of connection to the true prophecy,after he was shattered the prophecy was also lost, when everyone forgot gaster they also forgot the prophecy
14.The hammer of justice was a nickname given to Gerson by Asgore (& Gaster) and it just stuck
15.Alvin was half human, but due to looking more human he was left on the surface with other half human half monsters, and Gerson was not able to see him again & the reason Gerson has lived so long is because he is a boss monster, due to Alvin only being half human means Gerson only has to give half of the normal amount of soul magic
16.Sans and Papyrus are Gaster's great nephews, their parents died due to low HP and determination, gaster tried to raise them as his own
17.Gaster has holes in his hands because he was trying to defend himself during the war,
18.Gaster taught wingdings to Gerson and Asgore
19.Gerson is the only one who slightly remembers gaster, even then he only remembers that he once existed and now he doesn't, he cannot remember his name
20.Gerson, Gaster and Asgore are all Boss monsters
21.Gaster experimented with darkworlds so he, Gerson and Asgore out of fear they would grow apart but this eventually spiraled into a god complex
22.Gaster, Asgore and Gerson are all the same age, Gerson is just showing it because his son is still alive
23.Gaster & Asgore suffer from PTSD (such as gaster rattling his bones in defence/fear or Asgore freezing at the sound of weapons) for a few years after the war, Gerson usually helped them through it,
24.When they were kids they would make up stories about the Delta Warriors and their adventures
25.Gerson and Gaster always teased Asgore about his crush on toriel when they were teens
26.Asgore lost his parents from the war when he was only 16, Gaster and Gerson helped him lead the war
27.The war in general lasted about 10 years, starting from when Asgore was 15
28.Asgore still brews tea for three people sometimes out of habit
29.In the neutral route Gerson goes to Asgore’s funeral, being the one to spread his dust
30.In deltarune they were still friends as kids, just not as close as they grew up
31.Gaster is called “Dr.” Because of his many years as a field medic, and basically graduating with a medical degree from how long he served,
32. Gaster is very, very smart, his nickname from Gerson being “Egghead” when they were kids
33. Gaster made the gaster blasters as a tribute to Gerson due to them looking like turtle skulls and made the DT extractor as a tribute to asgore due to it looking like a goat skull
That's about it! I hope u like these headcanons! And if u wanna use them in your own stories, go ahead! Just make sure to credit this work pls