If you want to go the homemade route for the hot fudge, I love Smitten Kitchen's recipe. If you're gluten-free, simply omit the crackers or substitute another type of cookie. I've been really into the Schar brand vanilla wafers recently. Like, I ate a whole box in one sitting yesterday.
Heat a charcoal grill or indoor grill pan over a medium-high heat.
Spray a piece of tin foil with cooking spray or brush with vegetable oil. Place the foil on the heated grill and arrange the marshmallows in 4 rows of 4. Roast the marshmallows until golden brown, rotating every minute or so to make sure they don’t melt.
Meanwhile, place the crackers in a plastic bag and crush with the back of a spoon until finely crumbled.
Spoon the ice cream, one scoop at a time, into martini glasses, followed by two marshmallows, 2 tablespoons hot fudge, and a sprinkle of crackers. Repeat another layer. Serve the sundaes immediately.