Punnet square
Let's call black 'B' and white 'b'
Being intermediate, gray cats will have the alleles 'Bb'
Being recessive, white cats will have the alleles 'bb'
Gray
| | B | b |
White |b| Bb | bb|
|b| Bb | bb|
^Apologies for the awful drawing XD
Bb = Gray
bb = White
So the cats can have gray or white fur (A)