While Larry is probably right that intermediate treasury outperformed GNMA. What he didn't point out was that GNMA outperformed that benchmark bond fund (at least in this forum) Vanguard Total bond market. I believe (it's been a while since I ran the numbers) that GNMA's actually had less risk also.
The problem is like so many other things "past performance does not equal future success." Over the last decade+ we've seen very easy credit, over the last 3 years we've seen a remarkable rush to safety. So it is not at all surprising that "safest" bonds have been the best performing, nor that the almost as safe GNMA bonds would be the next best performing bond.
However, I and more importantly many bond experts like Bill Gross expect that the next 5-10 years will not be at all kind to Treasuries.
He raises good points but about the likely behavior of GNMA (i.e. pretty bad during rising interest rates due to people not refinancing cheap mortgages) although I don't think they did that bad in the late 70s and early 80s.