It'll automatically try to burn gas when it gets too old.

http://ask.cars.com/2011/07/will-the-volts-gasoline-stay-fre...

You could use a fuel stabilizer, like STA-BIL[1]. They are commonly used when storing gasoline for long periods.

[1] http://www.goldeagle.com/brands/stabil/

Eh, people worry about this but in my experience with storing vehicles is that it's really not that much of a problem. I put a motorcycle away for 1 year with zero prep. Next year, fired it up, ran the gas through (it seemed a little sluggish but nothing that can't be attributed to placebo), and filled it up fresh. And that's on a carburetor motorcycle with transistorized ignition, which is going to be much more finicky than a modern fuel-injected car with computer-controlled ignition.