Never understood that about the walking dead. Gasoline stops being vital after 3-6 months, so years into the apocalypse and everyone's driving cars, and Daryl has that sweet frankenbike. What are those things running on?
Gas is no longer in prime condition after 3-6 months, but is still combustible for at least a year or two. Old gas will damage your engine over time though. Most of the degradation is due to oxidization, so if the gas were in sealed cans, maybe it could last longer.
But yeah, 5+ years in and most gas is unlikely to work in an engine. You'd have to be making some kind of bio-ethanol at that point.
plot twist, the jeep is electric and there is a spinner-bike in the back you can use to charge it with. Only four short days of pedaling will get you 100 miles!