I would install an exhaust fan to pull fumes out and at the same time keep a window or two open when the generator is operating. That should eliminate any danger from carbon monoxide build up.

Storing the gasoline in your garage might violate terms of your homeowners' insurance policy and thereby void your insurance in the event of a fire. Storing the gasoline in the shed should not be a problem as long as it is not near the generator. You might check with your insurance company to be sure there are no problems with storing the gas in an outlying building. Also a good fire extinguisher rated to handle gasoline fires and mounted near the door would be a good addition.