Currently Online 
Yes. Just look at the pre-orders
Halo Reach (5 weeks): 1,071,366
Gran Turismo 5 (12 weeks): 143,898
Bit of a gap there, even if Reach launches 7 weeks before GT5. And then look where Reach was at at 12 weeks to launch
Halo Reach (12 weeks): 384,059
Thats nearly double what GT5 has at the same time prior to launch. Then compare GT5's precursor to Halo 3's expansion pack
GT5 Pronolgue: 3.83 million
Halo 3 ODST: 5.53 million
Personally I dont think the argument for GT5 is too convincing. id say its reasonable to assume GT5 will have better sales in the long-run compared to Reach, but then look at Halo 3. It had a massive launch which tailed-off fast, but managed to keep selling fairly well and eventually broke the 11 million mark, which looked like a long shot even after launch.
