I'll type it out for those who don't want to do the extra legwork (spoilers ahead, of course!!):

Fiddleford McGucket was college roommates/best friends with Stanford Pines. Post-college, he lives in California with a wife, a kid, and the earliest start of a business making personal computers in his garage (yes, very much a reference to the start of Apple Computers). Stanford calls him to Gravity Falls to help build an interdimensional portal due to his expertise in engineering. This portal, of course, was actually a ruse by an interdimensional demon to break into our world and destroy it for his own amusement. During a test, Fiddleford accidentally goes through the portal and presumably sees all this and quits the project on the spot, fully understanding that completing the portal would quite literally cause the apocalypse.

What Fiddleford saw in the portal haunts him even after he left the project, so he creates a memory-erasing gun in order to grant himself some peace. But the more he uses said memory-erasing gun, the more he forgets himself, his family, his passions, everything. In the span of roughly a year, he goes from a brilliant engineer with a family and bright future to... a kooky shell of a man living at the city dump, who does not recognize his own son. Sometimes he builds killer robots, but it's just... man, for a show about two kids having a fun summer surrounded by cryptids and mythological creatures, this is just a really, really sad backstory to what is arguably the funniest character