Per dictionary.com:
gray Audio pronunciation of "gray" ( P ) Pronunciation Key (gr)
adj. gray·er, also grey·er gray·est, grey·est
1. Of or relating to an achromatic color of any lightness between the extremes of black and white.
2.
1. Dull or dark: a gray, rainy afternoon.
2. Lacking in cheer; gloomy: a gray mood.
3.
1. Having gray hair; hoary.
2. Old or venerable.
4. Intermediate in character or position, as with regard to a subjective matter: the gray area between their differing opinions on the film's morality.
gray^2 Audio pronunciation of "gray" ( P ) Pronunciation Key (gr)
n. Abbr. Gy
- The SI unit for the energy absorbed from ionizing radiation, equal to one joule per kilogram.
grey Audio pronunciation of "gray" ( P ) Pronunciation Key (gr)
adj. & n. & v.
Therefore, I deduce that gray > grey