I agree you're probably in the clear. Base malt + Gamma lids = best case scenario for successful long term storage. Specialty malts would lose much more character, having more to lose; I'd be wary of those. (Crystal and roast can actually get somewhat unpleasant with age.) If you're on a budget and want to avoid waste, brew a batch. If you're happy with it, great. If you're not happy with it, then consider which you feel is more of a waste, spending your time, effort and other materials on making beer you're not happy with, or cutting your losses and ditching the grain. If the stored grain is not up to your standard, and you will no longer have regular opportunity to brew, then in the future, buy grain one brew's worth at a time. You'll pay more per pound, but waste nothing. And please share how it turns out, this will be useful, if anecdotal, information.