Fa gray cat and a white cat have kittens, what phenotypes could the kittens exhibit?

Depends on whether the genes that code for grey fur or code for white for are dominant, which ever one is dominant will show the phenotype. Assuming grey fur is dominant over white it also depends on if the grey cat is carrying white fur genes as these genes will be passed onto the kittens and with the white genes as well you may get a mix of grey and white cats. usually a punnet square is used to show this type of scenario.

.G - dominant g - recessive
     G     G
g   Gg   Gg
g   Gg   Gg  

This punnet square shows what happens if the grey cat has only grey genes, all the kittens will be grey as they all carry the grey gene,

    G     g
g  Gg   gg 
g  Gg   gg 

This punnet square shows what happens if the grey cat is carrying white fur genes, because of the white gene being present, half of the kittens would be grey and the other half would be white, even though one of the parents was grey it produced half non grey offspring because it was carrying white fur genes 

They both determine the exact age of rocks.