"was". If your referring to lifetime sales sure. But this gen alone, Halo sales as a franchise far eclipse that of GT.

Nope, It would be the 2nd full GT on PS3 and I think after GT5 not too many players still think Gran Turismo does what other racing games can't. And that is totally the case for Halo. Sure CoD outsells Halo these days, but Halo is still in its own league and offers such an great universe and so many features you simply can't find anywhere else. It really is something unique and GT5 has been surpassed when it comes to quality.

We're talking about a hypothetical GT6.

Not @sales: Someone said GT6 at launch on the PS4 would outsell it, and I think it certainly would, but then to make that a fair comparison we'd need either Halo 4 or another hypothetical, Halo 5, on Durango at launch, in which case I have no idea what would sell more.

Sorry I wasn't clear: I think GT6 would have very similar sales trends as GT5. I think it will be offsetted by that it wont be a big system seller as GT5 reeled that crowd in allready. Fixed my origional post.

But GT6 as a launch title would automatically limit its sales due to the console base just starting to grow. Unless Sony pulls a massive bundle process over the next couple years to offset it.

I doubt it would limit sales in any capacity:

On the PS2 

GT3 released to an install base of around 15M and went on to sell 15M.

GT4 released to an install base of around 80M and went on to sell 11M.

GT2 released 2 years later and sold 9M in total.

No for a few reasons:
1) Racing market is on the decline as seen by the sales figures for once popular franchises like NFS. Forza sales are predicatable/meh and GT5 was a big release for the PS3 (like MG4). Assuming that MG5 & GT6 would sell less than their predecessor.
2) Software sales for this generation are on the decline and this would be larger for the racing genre than a FPS game.
3) FPS are the most popular genre this generation
4) GT5 initial software offering hurt the GT brand.
5) GT5 has so much content (still coming) that some wouldn't adopt

First off Halo Reach and GT5 sales aren't even are fair comparison:
- Reach only sold in 2011 and 2012 due to extensive bundling with 4 other games. But since we do not know how much it sold in bundles we can't assume GT5 sold more standalone copies

- GT5 was bundled maybe once and is outselling Reach by a large margin on a weekly basis despite Halo Reach being bundled like hell.

Now for GT6 vs Halo 4:
We have no information at all about GT6 whereas Halo 4 is getting a large amount of hype at the moment due to its launch being only a couple weeks away. If GT6 comes too late in the PS4 cycle it will struggle to sell 7.5 million copies and

First off Halo Reach and GT5 sales aren't even are fair comparison:
- Reach only sold in 2011 and 2012 due to extensive bundling with 4 other games. But since we do not know how much it sold in bundles we can't assume GT5 sold more standalone copies

- GT5 was bundled maybe once and is outselling Reach by a large margin on a weekly basis despite Halo Reach being bundled like hell.

Now for GT6 vs Halo 4:
We have no information at all about GT6 whereas Halo 4 is getting a large amount of hype at the moment due to its launch being only a couple weeks away. If GT6 comes too late in the PS4 cycle it will struggle to sell 7.5 million copies and Halo 4 coming in at around this time should make it get 10 million copies.

A real comparison would be GT6 and Halo 5 as both launch titles and in that case there would be many factors for example:
- Which console is selling better
- The quality of the game etc.

So in reality this an unfair comparison that makes no sense at all
Note: To the people that are comparing Forza to GT lets start comparing Killzone to Halo

Stop overanalysing everything. We don't have all the facts, and that's part of the fucking point of a hypothetical question.

I love how I mentioned that a popular excuse was that Reach and GT wasn't a fair comparison and here you mention it. They are both "flagships" and both had a similar release window. What happened, happened.

Reach bundles were mostly digital and took place during the christmas 2011 season. Other bundles during non holiday periods werent as widespread as you think.