Hello and welcome to Under Falls. This is Gravity Falls, but with Undertale characters. That being said, it will probably be pretty similar to the actual show. But anyway, here's the characters:

Dipper: Chara (I normally imagine them as a girl, but in this thing they're a boy for two reasons: due to their role, and Undyne's role, Chara would have a crush on Undyne (and in this story Undyne isn't a lesbian) though that might not matter that much, but the other reason is because of the episode Dipper vs. Manliness. And if Chara was a girl it wouldn't work AT ALL for obvious reasons.)

Mabel: Frisk (I always imagine Frisk as a girl, and there's literally no way it could be bad if I make her a girl, so Frisk's a girl.)

Stan: Sans (he can take the author's name (Wingdings) if you want (but you have to request it first), and as for his explanation to trick people (since their names are fonts), his and his family's last name is Gaster, so that would mean he just used the aster font all the time instead of comic sans)

Wendy: Undyne (she's not a lesbian in this story)

Soos: Papyrus (he's Sans' twin (triplet) brother)

Fiddleford McGucket: Alphys (she has the last name Dino and her role comes from my headcanon that before W. D. Gaster fell into the Core, Alphys was his lab assistant.)

Gideon: Asriel (he acts like Flowey until he realizes he doesn't need to be a controlling jerk)

Robbie: Napstablook (edgier tho) (Napstablook is a boy, and I always imagine them as a boy, so he's a boy)

Pacifica: Muffet (she's famous because she's one of the few Spider's, aka the family who supposedly founded the first town to allow both humans and monsters (there's more now but when her ancestors founded the town there was none)

Ford: Wingdings (he's Sans and Papyrus' brother in this AU, and part of the Gaster triplets)

Gompers: Toby Fox (but he doesn't appear before Time Traveler's Dog and some things are different because of look below)

Waddles: Also Toby Fox

Candy: Monster Kid

Grenda: Catty (like, she still tends to talk like this and stuff!)

Sheriff Blubs: RG 01 "Rabbit"

Deputy Durland: RG 02 "Dragon"

Thompson: Jerry (Nobody likes Jerry)

Nate: Snowdrake

Lee: Chilldrake

Tambry: Shyren

Bill Cipher: Hypno Phobia (Hypnotale AU character (she belongs to Pinkie Fazpie, and if you check her profile, it references this story, so I can use her))

Same: Toby Determined, Shandra Jimenez, Mermando, Darlene the spider lady, maybe Quentin Trembley