Have you tried to blend and mix with its treat or food? This should work. You can mix little quantity with your gecko best meal and lets see how that works out.
The trick is dusting way lighter than you'd think. Most geckos spit out heavily powdered insects because they taste chalky and bitter. Try putting just a tiny pinch of supplement on 3-4 bugs, shake for barely a second, and feed those. You'll probably see an immediate difference. If your gecko still won't touch them, freshly molted superworms are incredibly soft and appealing—worth trying if you haven't already. What type of gecko are you working with?