Geckos cannot eat lettuce. Most terrestrial geckos, such as leopard geckos, are carnivores and eat crickets, mealworms and other insects. Despite their vegetarian nature, arboreal geckos, such as crested geckos, prefer to eat fruit and do not eat lettuce.
While geckos cannot be fed lettuce directly, many gecko keepers gutload their crickets and mealworms with lettuce and other vegetables before feeding the insects to the gecko. Gutloading involves feeding nutritious foods to insects that one plans to feed to the gecko. This boosts the nutritional content of the insect and makes it a healthier food source. Gutloading insects 24 hours before feeding and dusting them with calcium powder immediately before feeding provides best results.