GDE is an abbreviation of "genetic dead end". It basically refers to someone who will never breed and pass on their genes; hence a dead end (stoppage) of one's genes. The GDE usually comes to their position after interacting with an alpha male. This alpha male kicks him out of the herd or flock, and he's too meek to reenter another one. The GDE's circumstance is sometimes reached because the females of his species are able to detect that he has low quality genes. In ethology-related forums, GDE's are described usually as male animals, since hypergamy is rife in the animal kingdom too.
Among humans, males are twice as likely to becomes GDE's than females.