Here's the relevant map concerning the route study area for the GTA west corridor:
http://www.gta-west.com/pdf/6-11-12/5%20-%20Preliminary_Route_Planning_LARGE_June52012%2011X17.pdf
The proposed corridor would run both North and East of Brampton connecting to the 401-407 interchange.
There's a bit of a muddled tale how two separate highway proposals have been stitched together, so I think it's natural there's some confusion. As I understand it, here's a recap of the history on this over the past few years:
The province's initial assumption was that the GTA West corridor (separate project, has
its own thread here) would run east-west across the top of Brampton and Georgetown, punch through the escarpment, and head on for the north side of Guelph and potentially onwards to Kitchener.
With that in the background, Halton and Peel launched a joint study on what would be a north-south highway that would potentially connect with the GTA West route in a 'T', primarily as a means of servicing new sprawly subdivisions Brampton is planning for its west side. Although it doesn't sound like the province was really thinking much about it at the time, the hope on the part of Halton and Peel appears to have been that once the study was done they could hand the prelim work over to the province and they'd happily run with it as a new provincially-funded 400 out of the goodness of their hearts. If that didn't pan out, the idea of building it themselves a la the Linc in Hamilton seems to have been Plan 'B'.
What wasn't foreseen was that that partway through the province's study it concluded that the westernmost end of GTA West to Guelph was unjustifiable for the next few decades. The province redirected its planning for a crescent-shaped highway that would arc back to the 401. After a bit of futzing about with various unpopular options that swooped in through northwest Milton, GTA West seems to now be all-but-confirmed as landing on an alignment that overlaps the Halton-Peel corridor and basically replaces it.
The province seems to really be focused on designing the GTA West as a fast, uncongested route intended to first and foremost keep freight moving, while the Halton-Peel study seemed to assume a conventional local-traffic-friendly expressway that would support residential demand and let them zone for strips of office parks (sorry, "premium employment lands") along it. As they move further into design and start contemplating questions like how frequent the interchanges are spaced and whether development will be anticipated to hug it on both sides we might see some tension between those two goals.
Finally, there's no money set aside for anything yet, and some in Brampton would really like the north-south leg ASAP to move forward with expansion, so you could maybe see a situation where the municipalities clamour hard to phase the GTA West project with the bit they really want coming first, or maybe even make rumblings about fronting the money to get it started themselves.
