In reality you'll probably be fine w/ the 10g filter. it's not so much the size of the tank but the amount of waste that's going to be produced. I would assume your QT won't have more than 1 or 2 fish? If you have decorations/PVC
for the fish to hide in the bacteria will colonize there as well.
But if you insist, gravel would be fine since you're not really using it as a substrate but more as a confined pile to house bacteria. In that case, put it in your sump/tank for a week or 2 and then move it to the QT when you need it.. Instant bacteria !