If you're going to store gas in a locker, make SURE the locker "drains" outside, and there is no access to the cabin. You don't want gas fumes migrating into the cabin. Basically the same concern as a propane locker. And as others have said, there's a LOT of expansion in the sun, so if it's full, there's gonna either be fuel leaking out the vent, or a LOT of pressure inside!
Gas is a scary thing to be carrying on a small sailboat.
druid - likes diesel, and electric motor for the dinghy.