Add 3 cups water and 1 1/2 cups dry quinoa into a small sauce pan and bring to a boil. Cover and turn down to simmer for 8 - 10 minutes. When the water has been absorbed and the quinoa is nice an fluffy, add the rice vinegar, ginger and salt. Toss well to combine. Let the quinoa cool completely in the fridge while you prepare your filling ingredients.
Once the quinoa has cooled, place the sushi mat on a flat surface and begin assembling the rolls. Place one sheet of nori on the mat, shiny side down and scoop about 1/4 cup of the quinoa onto the nori. With wet hands, spread it out into a thin layer over the nori, leaving about a 1 inch strip uncovered along the edge farthest from you.
Lay the the veggies down first, then top with tuna and sliced mango. Lift the edge of the mat that is closest to you with your thumbs, holding the filling ingredients in place with your middle fingers.
Roll the mat over the ingredients, pulling back and tightening it after each rollover. Wet the uncovered edge and roll through. Gently squeeze the rolled mat around the nori roll and remove from the mat.
Place the rolls in the fridge while you assemble the rest of your sushi.
When you're ready, remove them from the fridge and slice each roll into 8 pieces, using a sharp, wet knife.
Serve with gluten-free soy sauce and wasabi paste.