Hi, Charles,
Minwax and Olympic both make Gray Stains.
I have used a mixture of Minwax Ebony and Driftwood stains to adjust the color of gray that I wanted.
There was also a tip given that you can use an acrylic craft paint thinned down for a wash but, on Baltic Birch I would use a top coat first so that it doesn't raise the grain.
The project I used the Ebony and Driftwood on was Bruce Worthington's Big Horn Sheep but, I don't seem to have a photo of it on my computer.
If you think a photo of that would be useful, I can scan it and e-mail the image to you.
Good luck and God Bless! Spirithorse