Hi,
I've never been to Cancun, but I don't think you'll find a gluten free store in Mexico - anywhere. You will, however, be able to find many naturally gluten free items in many places. You can have corn tortillas, tostadas, shrimp cocktails, fresh fruits, veggies, meats, tacos, salsa, pozole (yum), homemade ice cream, rompope (alcohol kinda like eggnog that's made and sold by nuns), jello with flavors like chocolate, vanilla, walnut, rompope, fruit flavors, as well as beans, rice, omelets, enchiladas (beware of the mole), etc.
Don't forget that you can bring Lara bars, dried fruit, and other non-perishable items (cereal, almond nut butter, rice cakes, hot cereal like cream of rice,...) into Mexico so that you will always have a safe snack. You can't bring dairy, fresh fruits or veggies, or meat into Mexico although jerky is usually ok.
There were a couple of other posts about Cancun on the travel board on here with very positive things to say. I'd recommend checking those out as well as any that talk about Mexico.
Have fun on your trip! When are you going? I'm going to central Mexico, in the mountains, for the last half of December. I'm just hoping it doesn't snow.

Seriously, it snowed once when I was there!
Ezme