GBS is the local beer-geek’s sign that the Holiday Season is upon us. It a fantastic beer. I’m glad you picked up some! GBS has become much more readily available over the last few years, though many of the GBS variants (six of them this year, I think) are still tough to find unless you’re hunting for them.
Beer - and more specifically high-alcohol, minimally hopped beer- that’s packaged properly won’t skunk*. Over time, it’ll eventually show signs of being oxidized, which would result in muted, cardboard-like flavors. A well-packaged big stout will last for five years or more, although beers with flavoring adjuncts (ingredients that aren’t grains, hops, water, or yeast) like GBS will tend to fall off in a couple years.
That said, barrel-aged beers will often get better with time, although barrel-aged adjunct beers (like Bourbon Barrel GBS) will lose some of the adjunct aspect over a couple years, but the barrel will result in the beer taking on really interesting (usually better) flavors over those few years.
* Skunking is the result of a photochemical reaction between hops and sunlight. Brown bottles help to forestall this reaction, but it can only work for so long. The “distinctive” flavor of Corona is from the fact that it’s highly skunked! Warm or extended storage doesn’t cause skunking, nor does going from cold to warm to cold, etc. Warm storage can cause hops in beers like IPAs to lose their punch, though.
You may well have opened a can of worms with a seemingly innocuous post asking a beer geek to explain some of the science of making beer!