Once Gloucester starts to hate Edgar and fully appreciate Edmund, the largest compliment he can give him is calling him "Loyal and natural boy"- as Edmund throughout the book is repeatedly called unnatural, implying that he goes against nature or is even a freak of nature, hinting that he should not exist. These direct attacks upon his existence (due to a fact out of his control) are then counteracted by his on father, the creator of this "unnaturalness", by finally deeming his existence to be true,
Edmund's validation is even further heightened by Gloucester as before this he says Edgar has an "unnatural purpose", meaning that Gloucester's reply insinuates that Edgar is not natural and Edmund in fact, is
