The official Dark Crystal website's encylcopedia lists 18 Skesis. It says there were two who were killed immediately after the Urskeks split into the Skesis and UrRu, and I'm not sure if those two are included on the list or not.I would think it be farther back than that. The one castle full of Gartham doesn't seem like enough to wipe out all the Gelfing in a short period of time. It has been a thousand years since the crystal cracked. The prophecy seems like it be something older than just one or two generations back....though we don't know how long lived a Gelfing is....
Infant male rescued by mystical monks. Female child hidden in hole...later found by living potato people.
I think they probably left most of the hunting of the Gelfling to the Garthim, while they sat back in the castle and watched.