They are chapter #666. Only the chapters #1 till #20 have primarchs.
The GK are definietely not one of the first founding chapters. The second founding is the most likely one, but there have been about 23 more foundings at least (so not all chapters who weren't made in the second have to be a first founding chapter ).
A forum community dedicated to Wargamers, Wargaming 40K enthusiasts. Come join the discussion about Warhammer and Wargaming collections, miniatures, tactics, terrain, reviews, accessories, history, displays, models, styles, scales, reviews, accessories, classifieds, and more!