"Best - These gutloading ingredients are best because they are highest in calcium, low in phosphorus, oxalates and goitrogens. They should be the primary components of your gutload: mustard greens, turnip greens, dandelion leaves, collard greens, escarole lettuce, papaya, watercress and alfalfa.
Good - These gutloading ingredients are good because they are moderately high in calcium and other vitamins/minerals. They should be used in addition to those from the previous category: sweet potato, carrots, oranges, mango, butternut squash, kale, apples, beet greens, blackberries, bok choy and green beans."
There are more but this list is the basics.
Cactus fruits, and figs, as well as grape and mulberry leaves are all good choices, being high in calcium. Most fruits are high in phosphorus and should be used in moderation. Squash blossom, hibiscus flowers, field plantain, green briar foliage, marigold petals, and nasturtium blossoms are also healthful and stimulating.