I've used gels that were a year out of date, and last year I fairly regularly used energy powder that was more than 2 years out of date (it was an enormous tub and I hadn't used it too often), and both were fine. The powder starts to clump together a little bit but it still dissolved, it tasted good still, and it still did the job. I didn't get any iffy effects.
I also found some gels recently that were 5 years out of date, I opened them up to have a look just out of interest and the glucose / sugar had started to solidify a bit. That was interesting I suppose, I doubt I'd consume one that old but it hadn't gone slimy or smelly or anything.
A few months out of date wouldn't ever bother me with these products.