There is something you can try to see if it really is an impaction. Pick the gecko up and feel the abdomen for hard masses. Just be careful because if you squeeze too hard you could puncture the intestinal wall and kill the gecko (if it really is an impaction). I am assuming the gecko is on paper towel now and you are monitoring its feces. You might find that the gecko can slowly pass sand out of the gut, little by little. As long as the gecko is warm and receives enough moisture, it should be able to eventually pass it. And since geckos eat sand all the time, even seeing it in the gut doesn't always mean there is an impaction.
Parasites can come from all sorts of strange places. People have speculated that they even get them from crickets. It probably isn't parasites, as you said, but anything is possible.
So I would suggest paper towel isolation, daily doses of electrolytes (salt + apple juice, Gatorade) or just water, warmth, and patience. I think your gecko will slowly drop the sand in its gut. It might take a week or so but you will notice the masses in the gut slowly disappear.