Fruits for Gutloading
Recommended fruits for gut-loading include apples, berries, pears and melons. Some reptile-keepers gut-load with more exotic fare like figs and prickly pear. Generally, sugary and mushy bananas are avoided, though you can offer your insects banana peels. According to the Lizard Lounge, "some geckos" will eat fruit directly. Try papaya and mango, which are native to the gecko's habitat.
Vegetables for Gutloading
Leopard geckos require plenty of calcium, so gut-load insects with endive, turnip greens, dandelion greens, collard greens, mustard greens and watercress. Kale and spinach, though high in calcium, contain high phosphorous and should be used in moderation. Potato, carrots, sweet potato, parsnips, squash and yams also work for gut-loading. Lettuce-type greens are low in nutrients and should be used sparingly.
How to Gut-load
According to Pro Geckos, you should gut-load insects for at least 24 hours prior to feeding the insects to your leopard gecko. Offer insects a variety of foods rather than just one or two kinds of food.
How Often to Feed
Leopardgecko.com recommends offering two appropriately sized insects for every inch of a leopard gecko's total length every other day. Insects offered should be no larger than half the width of the gecko's head to prevent choking. A well-fed gecko has a fat tail.