Obviously, it's more challenging to do gray on red oak than white oak due to pink undertones on red oak.
We generally don't use the premade stains for gray, but rather use a mix of white and ebony (from bona) and you often need to go darker on red oak to mask the red tones.
Bleaching the floor first is also an option, but not one I would recommend.
You can see some picts in this article:
http://theflooringgirl.com/blog/stainin ... ty-ny.htmlNote: this was written for customers (not installers), but the picts of the mixing ratios may help. We recently did a home where a customer had red oak on half the floors and white oak on other half so we tested on both.
Debbie, The Flooring Girl
http://TheFlooringGirl.com