Simple math would dictate that Halo Reach will outsell GT 5 by a fairly large margin barring GT5 getting bundled at some point.
Halo 2 sold 8 million on an install base of 25 million. GT4 sold 14 or so million on an install base of 130 million (roughly).
Halo 3 sold almost 11 million and it launched to an install base of around 18-20 million.
With the current hardware numbers for Sony and MS I don't see how GT has any shot at beating Halo this gen....unless of course it gets bundled.