My recommendation/suggestion would be to send it ASAP to the nearest Fuji repair center. Or at least ask them for their recommendation/suggestion.
Maybe still covered under warranty or repaired under goodwill since advertised (?) as water resistant or water sealed.
That's what I'd do. You're in luck as rain water is not full of corrosive salts, but if there's some water in there, it's best to get it opened up and cleaned out as quickly as possible.
BTW: Many/most electronics can survive a dousing and dunking in rain water without problems, even operate perfectly while still wet. Flipping the power on may actually help, by heating up the circuitry and little it evaporate the water more quickly rather than letting a little bit of water to sit there, hoping for silica gel to draw it out, and slowly allow oxidation to occur.
Getting in touch with Fuji service is the first thing I'd do ... get their recommendation, cost of a service and cleaning, etc. Having to have a camera and lens serviced, particularly an expensive camera like a GFX, may cost a little but isn't the end of the world .. or the camera.
G