Drain it. fresh fuel and StaBil. Or your choice of fuel storage additive.
Run it up with a good load on it and get the oil hot. Then put it away.
Here, allow me to blow your mind...
If you convert your genny to LP, where-ever you find an unclaimed grill tank you have fuel to run your genny -- Gasoline not required... It's what I run mine on.
https://www.motorsnorkel.com/