I haven't actually tried them, so I can't comment on the taste, but the ones I have heard of are the Stay Alert gum that is military issue and Jolt gum. I have seen Stay Alert sold on Ebay for roughly $1/pack (5 pieces/pack). One piece of Stay Alert has the caffeine of one cup of coffee, and Jolt gum about half of that. Since Jolt is a regular consumer product, I wouldn't be surprised if it tasted better than any military issued gum.
Although I have seen Black Black gum, made by Lotte and sold in Asian food stores, advertised as a caffeine gum, each stick only has a tiny fraction of the caffeine found in one stick of Stay Alert or Jolt gum, so not very useful unless you are very sensitive to caffeine.