Note, these roles loosely fit into the original UT's. This is technically more of a Gravity Falls AU than an Undertale crossover.
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Dipper -> Frisk
Bill+Stan -> Chara
Wax Stan -> Flowey
Ford -> Asriel
McGucket-> Toriel
Mr. Northwest -> Asgore
Wendy -> Sans
Wendy's Friends -> Papyrus
Blubs + Durland -> Undyne
Stan + Soos-> Alphys
Gideon -> Mettaton
Mabel -> New Role
Stan -> New Role
Bill -> New Role

The UFO -> The Ruins
Forest + Cementary -> Snowdin
Enchanted Forest? -> Waterfall
Mystery Shack -> Lab
Mystery Shack basement -> True Lab
Forest?? -> Hotland
????? -> Core
Pathway to + Northwest Mansion -> New Home
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A few somewhat small story elements to note:

Everything before the incident with Ford being trapped in the portal would be the same in this universe. When Stan and Ford did end up meeting, Ford was still forced into the portal, but the anomaly became large enough to solidfy the barrier around Gravity Falls that traps the Weirdness in. The only to break this would be to gather the people of the Zodiac (Cipher Wheel). At least, that is Stan's theory. Mr. Northwest learns about the Mystery Shack and, thinking that Stanley is Stanford, hires him to crack the barrier and allow Mr. Northwest to continue his business outside of Gravity Falls. Stan agrees (and charges the Northwests quite a bit) and gets to work studying the weirdness that his brother had recorded, while also running the Mystery Shack in the process. Soos eventually becomes Stan's assistant.

Mabel and Dipper are still being sent to Gravity Falls to stay with Grunkle Stan, while the parents aren't aware of the one-way barrier surrounding it. The two twins accidentally become seperated on the way there, ending up on opposite sides of the town. Mabel was captured by the local people and brought to the Northwests, as everyone is slowly growing desperate. Dipper finds the final third journal at the very beginning and ends up lost in the UFO. There he meets McGucket, who was harvesting the place for parts.

Wax Stan was made sometime during Stan's career. Stan tried to use it as a "host" for his brother's mind, but in activating the portal, it only brought back a fragmented copy of it. Wax Stan could hardly be considered Ford, only having his brains and a hostile attitude towards everything. Stan tried to reason with the clone, but Wax Stan managed to run off and disappear.

The people and creatures of Gravity Falls are after Dipper not only because he has the last journal, but because he soon earns the pine tree hat, making him the final person needed for the symbol. The Northwests would pay handsomely to catch him, and everyone wants the pay. He wouldn't be captured in the end, but would still be going after the Northwests to find his sister.

In the theortical genocide route, it wouldn't really be killing. Dipper would meet Bill early on and not knowing his true intentions and powers, would make a deal with him. Bill's plan would be to unleash Weirdmaggedon on the town and get rid of the Zodiac people in his way, starting with McGucket and ending with Mabel.

Instead of fighting Ford at the end of the paci route, you would fight Stan. At the end of the True Lab, the portal is almost going to activate. Stan is blocking your way the entire time with various traps and battles to slow you down, and in the end is ready to full on brawl. The portal then activates, the anomaly big enough to shatter the barrier around Gravity Falls completely, and the real Ford coming through the portal. Mabel and Dipper reunite a little bit before this.

Gideon is the shining star of the town, but is still Stan's rival. Gideon had heard about Mabel and started to fall in love. Dipper was digusted by this and tried to get information out of him, but unsuccessfully until the very end when he almost outed Gideion as a fraud.

Blubbs and Durland are doing their job as policemen in the Waterfall section, trying to track Dipper down. They aren't that good at it.

Wendy and her friends are doing pranks in the forest when they meet Dipper, not knowing he's a symbol and not really caring either way. When they find out, Wendy's friends are ready to capture him immediately, but Wendy protects Dipper.
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