Storing gasoline in a shed is OK as long as the shed is vented or at least you have no other ignition sources in the shed.
You should not store gasoline though in an attached garage especially if you have a gas water heater or other ignition source.
If you do have an outdoor shed that is detached from your house then you can safely store gasoline in the shed.
I keep a couple cans of gasoline in 5 gallon cans in my outdoor shed for my lawnmower and weed eaters and other outdoor equipment and I've been doing it for years.
Never had an issue.
But if you do store gasoline make sure that you ventilate the shed if it already isn't especially during the summer months.