Do you know what color the mares base coat was before she turned gray?
Okay here what I figured out.

1. With out knowing anything background wise, just going off the mare being gray and the sire bay these are your chances= 75% gray and 25% bay

2. Now if the Stallion is unknown for the red factor and Homozygous for the Agouti gene(bay/black points) and we leave the mare unkown on the red factor and the Agouti and she is Homozygous for gray your results are = 100% gray

3. Stallion is unkown for red and Homozygous Agouti and the mare is unkown for red factor and Agouti and is heterozygous for gray chances are= 50% gray, 46% bay, and 3% chestnut.

But just plain on having a gray baby. These foals as you know are born bay, black or chestnut and turn gray. When the foal is born check around eyes and muzzle and on legs for random white hairs. If you see these good chance baby will turn gray.

Good Luck