In a pot over medium heat, combine pork and water. Bring to a boil, skimming scum that floats on top.
When the broth has cleared, add onions and fish sauce.
Lower heat, cover, and continue to cook for about 1 to 1 ½ hours or until pork is tender. Add more water in ½ cup increments, if needed, to maintain about 6 cups.
Meanwhile, cut into guavas into halves and using a small spoon, scoop out the seeds.
In a bowl, combine guava seeds and the remaining 1 cup water. Using the back of a spoon, mash to extract pulp and strain in a fine-mesh sieve. Reserve the guava juice and discard the seeds.
Trim about 2 inches from the kangkongs stalks and discard. Cut kangkong into 3-inch lengths, separating the sturdier stalks from the leaves. Wash thoroughly and drain well. Set aside.
Add gabi, sliced guava, and the guava juice to the pot and continue to cook for about 5 minutes or until tender.
Season with salt to taste.
Add kangkong beginning with the sturdier stalks and then the leaves. Continue to cook for about 1 to 2 minutes. Serve hot.