Here's a sales comparison of the latest GT vs the latest Halo:

The last "main entry" or numbered Halo, Halo 3, hit 11.60M, was a current gen game and is the best selling game in the series. GT5, the best selling current gen GT, is a few million off this number and though it's expected to sell at least another million, it still won't get close to 11.6. Gran Turismo has been known to sell up to 15M (GT3), but that was all the way back in 2001 on the PS2. 

So, let's say GT6 was releasing this year on the PS3 of course, would it have a chance at outselling Halo 4? (Let's also say they have similar marketing in terms of budget and quality).

Source (sales data)

It's very possible because Gran Turismo is a bigger franchise than Halo.

Is it? GT has declined in sales since GT3. Each main Halo game has sold better than the previous, though if you count Reach, Wars etc. it fluctuates. 

A better question is "could GT5 outsell Reach???"

Anyway GT6 could outsell Halo 4 if it was launched this year... 50% chance.

No. Gran Turismo is on a decline right now. Gran Turismo 5 also got pretty mixed reviews and hurt the series because of that. While it would sale a lot as usual, not enough to beat out halo.

No. Forza is stealing too much thunder from GT, and racing games in general seem to be on a downturn since last generation.

It remains to be seen whether Halo truly gets its thunder stolen by Call of Duty or not, but if the pre-order chart is anything to go off the answer is probably 'no'. The games seem to be able to live separately by being different enough in style that people are inclined to buy both.

I don't expect we'll see GT6 for the PS3 though. GT5 is pretty much just a money-printing machine for them at the moment with still a fair amount to offer, and GT6 is probably too important for getting PS3 out of the gate. Or Vita off the ground.

It is possible, but I doubt it would.

No I dont think so. Racers in general seem to be selling less now days while Halo 4 is the first true "mainline" numbered entry since Halo 3.

I dont think so either. Note iam a PS3 fanboy. If it were a release title for ps4 yes definitely. But for the PS3 never. Gt has legs but this late in its lifecycle this wont help much.

People made "will GT5 outsell Reach" in 2010.

Halo Reach smokes GT5. Then you get all the "Halo Reach and GT5 arent even the same genre", neglecting that the comparison was flagship vs flagship. Also the ever popular "Reach released in September and GT5 in November, its not fair!" Ah fun times.

GT releases too far apart now to compete with Halo franchise sales. But even as a 1 on 1 comparison, Halo 4 is a true sequel where as Reach was a main title albiet with different characters. Bottom line, if GT5 cant beat Reach, then GT6 can't beat Halo 4.

EDIT: Also since GT5 got mixed reviews and Forza has been 90+ metacritic and releases 3x as fast, that has begun to shatter the notion that GT is the ultimate game in the genre.