Gork and Mork were most likely two of the Old Ones the Orks began worshipping as gods during the end of the War in Heaven (the Orks were more made to combat the growing number of hostile warp entities than anything else) and the ensuing Enslaver Plague, when the original old ones died the belief in them remained and either spawned two new gods, or more likely the collective faith in them helps shape the Orks manifest and perceive their latent psychic field, hence Weirdboyz having a power that manifests WAAAGH!!! energy into a giant foot and similar things.
My headcanon is that the beings who the Orks referred to as Gork and Mork were the same as the ones the Eldar called Khaine and Cegorarch, but when the real things died, as all Old Ones did, the two races created new ones from their psychic energy and according to their own perceptions of past events and social norms. The Eldar having been around before the Orks had more Old Ones to reinterpret in their pantheon, while the Orks only met the ones who lived until the end of the war and perished in the Enslaver Plague that followed.
So the original Khaine/Gork was an Old One general who lead the Eldar in the War in Heaven, but was corrupted by Necrodermis after a battle with the Nightbringer (actually part of Khaine's fluff) and became more callous, cruel and bitter over time as a result. Eventually he became the cruel and monstrous figure portrayed in Eldar mythology, and was for some reason one of very few Old Ones to survive to the end of the war. When the Orks were created he was given command of them, as it was logical to put a rage fuelled monster who thrived in hand to hand combat in charge of the Orks who shared his proclivities, if not his motivations. He then perished in the Enslaver plague and posthumously served as the inspiration for part of the Eldar and Ork pantheons.
Cegorarch meanwhile was presumably an Old One specializing in more subtle and secretive methods of war, snipers, disguises, assassinations, ambushes and similar. Originally leading Eldar scouts, sniper teams and saboteurs and fighting the Necrontyr with deception, misdirection and attempts to turn them against one another. During the creation of the Orks he attempted to instill in them the same tactics, leading to the presence of the latent gene combination that creates Kommandos, and possibly the entire Snakebite clan, and then lead them post creation in some battles, instilling in them the concept of Mork. Like Khaine upon his death at the hands, or tentacles, of the Enslavers he was the foundation of new gods representing his ideals in a loose sense.