t'is i, the one who said i had steven falls over headcanons a few hours before (had to get two hours of sleep to rearrange my head first actually).
so i was thinking that at first mabel and dipper were tasked by bill cipher (their boss) to go and investigate earth to see if the natural resources were good enough for their home planet and speculatively most of it were located in gravity falls but the twins had to disguise in order to avoid suspiscion from the earth’s government (who probably would waste more natural resources if they were alerted) and disguised themselves to live among the humans. thus they met the two stans (who were working on a portal based on the “gem anomalies” happening there, the portal is actually a warp pad that could be a gateway between earth and the gems’ home planet but it isn’t complete yet) and lived among the two.
while mabel was seamlessly adaptable because she was a pretty rebelious gem, dipper adaptation process was long and he would secretly read up the stans’ research to sabotage the making of the portal so that humans don’t find out about the home planet. but time went on and both dipper and mabel ended up being pretty attached to the stans and the citizens of gravity falls and at one point didn’t dare to give a vital information to bill because they didn’t want bill to destroy gravity falls (and earth).
meanwhile, amy was one of the higher ranked gems and heard about what bill was planning and didn’t like the fact that they were taking natural resources from a planet that still needed it (maybe so far the home planet gems just took it from planets without living beings like humans) and decided to rebel. wirt used to be a technician class sort of gem who used to be under mort that was pretty undervalued until amy came in and liberated him, she invited him and a handful of gems to rebel against the main government of the gems (and the face is bill cipher, maybe with those other shapes the gf fandom had thought about like tad strange) and went to earth (situated in the outskirts of gravity falls but the opposite side of the mystery shack).
mabel and dipper heard about the coming of amy’s army and decided to give their knowledge of earth to help her win the war and thus they met wirt too (who used to not stand any other gem except amy, thus finding how dipper and mabel are already in touch with earth’s culture as annoying or smth).
at first they refused to leave the mystery shack because they thought of it as their home but one of the stan twins (probably grunkle stan) was killed during the war and mabel and dipper knew that they needed to get away from the humans for a while if they wanted them to be safe.
the war ends in amy’s victory with lots of casuality from both sides (thus a handful of monster gems that added into gravity falls’ anomalies) and the portal being destroyed. the stan twin who lived ends up having a lineage of family with dipper and mabel feeling responsible to protect them so his lineage referred those two to something like guardian angels but dipper hid stans’ research from the lineage.
on amy and wirt’s side they didn’t interact with the residents of gravity falls as much as dipper and mabel did (more of admiring on the sidelines in amy’s part and more of disgust in wirt’s part) but amy met human jonathan and fell for him and ends up destroying her current physical form to have greg.
though wirt spited the boy, he is very protective of him because the gem belonged to amy before. mabel knew that it was pretty toxic for wirt to be overly protective like that (especially with /that/ reason) and with jonathan, tries to introduce the young boy to earth’s culture and is pretty lax with him - acting like an older sister instead. the one who teaches him about gem culture (reluctantly at first, but mabel insisted) is dipper and wirt (once he has warmed up to greg though he ends up getting more homesick than expected) and the two boys ends up acting like greg’s (gem) parental figure.
and thus the four of them go on missions to bubble up the defective gems and learn more about the why gravity falls seems to have more potential for gem power than any other places in earth whilst teaching greg on how to control his gem powers. but little did they know that this generation of stan pines’ lineage (who might’ve been twins again with the same name as the ancestor who met dipper and mabel?) seems to have uncovered the two stans’ research and has the intention of turning on the gateway/portal to homeworld without their guardian angels knowing. and maybe, just maybe this would open up the chance for bill to get even with the rebels… (or reinvade earth? i dunno)
wow this took longer than expected. i was expecting a few asks but this turned into a submission. and there’s still a lot of plot holes anyways like where does the other gf and otgw crew fit in and all especially since i just basically made a “reincarnation” of the stan twins to be greg’s friend. this is probably more gf related than ever but i really don’t know how to fit the unknown in this, yet. so enjoy reading this mass of headcanons and feel free to add some of your own!!! (maybe i’ll return more when i’ve thought out about it more) and hope you stay healthy skim!!
and i cross my fingers so that you would receive this submission, amen.