California Greys are purebreds. They're not F1 hybrids, so they will breed true.
They were bred in the 1930s from crosses of Barred Rocks and White Leghorns. They're sexable by color because all barred/cuckoo birds are sexable by color (males lighter colored, females darker). Both sexes have white dots on their heads when they hatch. If you look up how to sex Barred Rock chicks, the same info applies to these.
Here's some links with info about California Greys:
http://www.feathersite.com/Poultry/CGA/CalGray/BRKCalGray.html
http://en.wikipedia.org/wiki/California_Gray
http://www.raising-chickens.org/california-gray.html
California Grey rooster x White Leghorn hen = California White, which is sexable at hatch by feather sexing (which is different from a sexlink). In color, they are white with the occasional black feather. This may be the hybrid people are thinking of...
Hope this helps!