There are only two base colors, red and black. And any other combination of patterns and modifiers all revolve around those two base colors, it's the same for all horses.
Gray can modify either black based or red based horses. So, like others have said, we'd need to know the actual color of the grey. Also, to get a bay, there has to be an agouti gene coming from somewhere(bay is black based with the agouti gene). The fact that the cross produced a chestnut really doesn't tell us anything, because you already told us one was a chestnut(red based) so there's an automatic 50% chance of producing a red based foal. Agouti can hide in red based horses. So your two horses might both carry it or neither may carry it. If neither carry it, you can't get bay. If your gray is red based, you can ONLY get red based foals. RedXRed produced ONLY red. Now, depending on the modifers it could be red hiding agouti, palomino if the grey carries creme, red roan or red pinto patterns if the grey carries any of that or silver chestnut if either carries silver.
If the grey horse is black based, and heterozygous for black, than you could produce black based foals, but can also produce red based foals, and any combination depending on the modifiers. If the grey is homozygous for black, then the cross will only produce black based.
Also, when a parent is grey, there is a 50% chance that the offspring will be grey, which will hide any color eventually anyway, so a lot of people don't like to use them for breeding, especially pinto breeders.
