I'm not especially inclined to give guns a bunch of rules different than other ranged weapons. But I think weapon mastery properties could be used to give guns a flavor of their own.
That's fair. You simply set up the narration that guns aren't crazily more effective than medieval/fantasy weaponry, people on Earth simply don't (or didn't) enjoy any sort of supernatural resilience. A 10th level fighter can take a few bullets just like they take sword swings or claws.
Drive and Computers, maybe. What else do you feel is necessary, and what could be tool proficiencies trades, I think)?
Tools could definitely work. Land vehicles becomes medieval land vehicles. Then have tool proficiences for autos, motorcyles, large vehicles, planes, etc.
Some skills would need to be context-specific. History for a fantasy world refugee wouldn't necessarily include knowledge of ancient Egypt; likewise, History for an Earth human wouldn't cover the War of the Lich-Kings. That would depend on the relative recency of the fantasy world migration, though. Big difference between "last year" and "50 years ago".
If the focus is more on Earth humans as PCs, I might have more Int skills that focus on broad Earth-relevant topics. Science and Law come to mind. "Computers" could be a skill, or could be a tool proficiency.
Maybe classes and subclasses can be divided up between Mundane (people from our world with no connection to the fantasy world), Touched (people of Earth who have connected to the Fantasy world*) and Supernatural (people from the fantasy world*).
Good question.
*This is based on a setting I have used in previous games and am writing fiction for, in which the people of a very D&D-like fantasy world flee the destruction of their world to Earth through portals.
Really, this comes down to a worldbuilding question. Ideally, I'd take the main magical classes (and magical underpinnings) and decide how they work on Earth.
Can normal Earth humans level? Can they learn wizardry? Does divine magic work on faith, and is thus open to the faithful of Earth? Can a talented musician gain bardic magic?