I think we all can agree that besides obvious differences between the two shows,  they share weird chaotic energy (especially DGHDA season 2), so I decided to list things those two have in common. SPOILERS AHEAD

chaos, chaos, chaos

time travel

weird cult using some Sci-Fi machinery (Man Of The Machine, Society Of The Blind Eye)

portals 

COLOURFULLNESS

a small town with regional gothic vibes where weird things happen (Bergsberg, Gravity Falls) 

magic/science fiction stuff

siblings! + angst/hurt/ making mistakes/one is trying to fix everything after they did something wrong but in the end, they love each other 

family/ found family/friendship feels

main characters are the chaotic energy colourful goofball (Dirk and Mabel DO share similar vibes) and plainly dressing nervous nerd considered loser by some people (Todd, Dipper) 

also Mabel sweaters - Dirk’s jackets anyone????

both Dirk and Dipper are technically detectives

keeping secrets from the rest of the family

different dimensions/universes - (backstage of reality, Wendimoor + all the GF dimensions)

aggressive villains with a Southern accent (Gideon Gleeful, Osmund Priest)

secret government agency getting involved

not very responsible, police comprised of softies that often do not-police work while on duty (I'm thinking GF police and Tina and Hobbes, cause Zimmerman  and Estevez actually were effective; also Tina and Hobbes in the end actually were a good policeman, but Gravity Falls and Bergsberg department actually share similar vibes so...)

music festivals (Woodstick Festival, Sound Of Nothing) with love spells (Suzie Borton miscast spell, the Love God shenanigans in GF) 

books with magic spells (the Wendimoor book & the Pines Journals) 

animal sidekicks (Waddles and Rapunzel + the Kitten-Shark)

switching bodies (Machine in DGHDA and the Body Swapping Carpet in GF), also with animals (Lydia-Rapunzel; Soos-Waddles)

a lot of illegal stuff happening and a lot of hurting happening

incredibly cool but actually nervous warrior-female that's handy with weapons (Wendy, Farah) that the anxious nerd (and sometimes loser) has a crush on (Todd, Dipper)

unicorns exist but are kinda weird (they were unicorns in Wendimoor, just reminding ya)

scientist inventing weird machines (Patrick Spring, Fiddleford and Stanford) 

weird oracle-magic-humanoids  - Wakti Wapnasi has the Oracle vibes (Oracle is an entity from one of the GF dimensions mentioned in The Journal 3 that Stanford met during his travels) 

weird houses/mansions where weird stuff happens (Northwest Manor, The Mystery Shack, Cardenas House, Patrick Spring mansion)

(this is a bit of a stretch but oh well) music Bands (Sev’ral times and Love Patrol Alpha in GF  The Mexican Funeral in DGHDA) 

shapeshifters (also kinda a stretch, cause Mona isn’t exactly a shapeshifter) - Mona Wilder & the Gravity Fall’s shapeshifter 

weird guns (Stanford’s gun & the airgun in DGHDA) 

the motive of “taking responsibility” and “taking action”

psychics that aren't psychics (Gideon, Dirk) 

brass knuckles (Todd has one’s in s2 + obviously Stan’s brass knuckles)

angry (punk) people (Rowdy 3 + the Gravity Falls Girl Squad in The Last Mabelcorn episode, the bikers from Gravity Falls’ biker bar called Skull Fracture and of course, Stanford Pines)

girl power

weird symbols and tattoos 

humanoid magic creatures (Wendimoor creatures + all of the supernatural creatures from Gravity Falls)

the rift in the fabric of reality in Gravity Falls has similar vibes to “the universe is broken” trope in DGHDA

retro/vintage vibes sometimes 

retrospection