If you are looking for the word "grey" exactly how you spelled it in the question, the book uses it exactly zero times. If the alternate spelling, "gray," is acceptable, then the author uses the word 22 times within the text of A Separate Peace."
The page numbers are as follows: 3, 12, 22, 43, 47, 50, 53, 57, 59, 68, 69, and 72.
That is not 22 different pages though. The reason for that is because John Knowles sometimes uses the word "gray" multiple times per page. Page 50 has two occurrences of the word. Page 68 has three uses. Page 22 has a total of seven uses. Most of that page's uses are contained within just a few paragraphs too.
I have one caution though. The page numbers of my version may be different than the version of the book that you are reading, so those page numbers might not match up exactly.