graham crackers: we use kinnikinnick's "s'moreables", and they're really accessible - superstore even carries them if you're not near a specialty natural foods store. they are nut free, dairy free, and gluten free so some of the great bases are covered!
marshmallows: we use kraft, and while their labelling policies have just recently changed for the worse, we've been told that the marshmallows are still safely gluten free. (if you've been trusting kraft's policy of declaring any source of gluten in the ingredient list, now is a great time to brush up on their new policies!) beware of no-name brands, make sure you check with the company before packing them along on the trip!
chocolate: we used our favorite dark chocolate from giddy yoyo, partly because we love it and partly because it's dairy free. if you handle dairy just fine then you have lots of options - check out hershey's new and improved gluten free list, it's really impressive and will point you towards some options for your s'mores! (as well as some options for your ice cream sundaes and road trip snacks)
last but not least, make sure your roasting stick is free and clear of the dreaded g! we always bring our own when we have campfires with friends, and while we're up for sharing everything else, we keep those to ourselves.
s'more away my friends!