Does the Gregory Whitney come with a rain...
Does the Gregory Whitney come with a rain cover and if not can you get one to fit.
The Whitney does not come with a rain cover, and although the fabric is PU coated, rain will certainly soak through the seams, so you will need some sort of weatherproofing.
One option is a rain cover, I've provided a link to Gregory's, but there are a bunch of different brands and models with different stats. You're going to want the Large for the Whitney.
Another option is a pack liner, which is lighter, 360º weatherproof, and more durable because its protected by your pack. The downside is it you have to set it up before the rain, and it limits how accessible your gear is. This one will fit most of the Whitney.
The third option is to use a trash bag as a pack liner. This is light, cheap and versatile (could be for solar stills and emergency shelters), but isn't as environmentally friendly or slick. You'll want a heavy duty yard bag or, even better, a 3 Mil Contractor bag like this one
Hope that was helpful