What do you measure ?
The trouble with this sort of report is that they measure volume in terms of dollars. This weighs an expensive operating system more than a cheaper one and both more than one that is free. So how should you count: number of new installs, the amount of use (whatever that means), transactions, or ... ?
Distro watch works by counting visitorsOK for desktop operating systems, and puts Ubuntu at about 6 times RedHat: http://distrowatch.com/stats.php?section=popularity
What about the operating systems that you dont see, the ones in your broadband router and other embedded systems ?
But Gartner is paid to do research by the big boys, the ones who sell operating systems, so it is natural that they will want to make their customers look good.