First for the good news
First for the good news regarding the 3.0 I6 Duramax. GM will have several advantages, especially if this program is successful. To understand the advantages, one first has to look at the competition.
Nissan has a financial relationship with Renault Motors of France, who would undoubtedly be more engineering capable of coming up with a good diesel for a half-ton truck that could keep it out of the 3/4-ton weight limit than what they currently have with Cummins, but from a cost standpoint, and a utility standpoint, the relationship with Cummins gives them little to compete with. A Renault-designed diesel would have limited production capabilities unless they set up manufacture in North America, which would be a huge cost undertaking and very risky if they never meet volume numbers and Titan has never really managed to make much of a dent in this segment. As for Cummins, as far as I know, other than a six cylinder that Cummins worked on with the EPA in the first part of the millennium at the same time that they were working on that 5.0L V8 that did come to fruition, the only other engine Nissan could source from Cummins would be the 2.8L four cylinder that has been rumored to show up in a new mid-sized Frontier that would not work for the big Titan.
Toyota has little or no diesel know how. They'd have to source one. That would put them at a cost disadvantage versus GM and Ford and possibly even Ram. I don't think Toyota will even try the diesel route; rather, they may produce and market a full-up hybrid, because they know that arena much better than they know diesels, and due to their weakness in diesel tech, they'd be able to offer a hybrid with similar results cheaper than they could a decent diesel choice.
Ram sources, or has sourced before the ban, a 3.0 V6 Ecodiesel from VMI of Italy. There is rumor of Fiat working on an inline diesel design in Italy in the displacement range of 3.5-4.5. If this happens, Ram would be able to better compete, however, it would still not put them on par with GM, because GM will have domestic production capable of ramping up or down very quickly and will save on shipping costs versus a European-made inline diesel for the Ram.
Ford is in a position much like what Ram might be if they get a Fiat-built diesel 1/2-ton engine, but compared to a possible upcoming inline Fiat engine and an all-new Duramax inline 6, the Ford PSD will be an older design and is a slightly more expensive V-type motor. The 3.0 PSD was originally built by Ford that sprung from a partnership with Peugeot in the early part of the century, which was absolutely state-of-the-art technology for it's time, because Peugeot is one of the diesel kings of the auto world, but will show it's age once GM comes forth with an all-new Duramax.
So GM's diesel endeavor will have the highest upfront commitment and origination costs, but in the long run, they'll have advantage if they actually produce and sell some volume. The advantage of in house design and domestic production, and the advantage of being able to design and build the latest and greatest from what technology has to offer, and a big part of that, especially as it relates to expensive diesel manufacture and emission certification, is the possibility or cost savings. Nowadays, solenoid injectors are back, but GM is the only one that has them in our market; they are cheaper than piezo, and can now once again meet emissions standards. GM has also utilized cast iron block designs in their latest diesel offerings, which has a cost advantage over GCI, if they can package it at weight low enough to make it competitive performance wise.
Now for the bad news...As stated above, GM has found some cost savings in some of their latest diesel products. GM is now the light-duty diesel leader for North America; sort of taking up the hole left by VW. They have a 1.6L that they offer in three vehicles. It's cast iron; it has solenoid injectors. They also have a 2.8L Duramax offered for a lot of extra $$$ in the Colorado and Canyon pickup; it's cast iron block; it has solenoid injectors; it's built in Thailand. But despite the cost advantage it would seem that GM has found through reduction of production and material costs, GM has not shown so far that they are willing to offer any more value to the customer than if they had not found those savings. And so my prediction, based on GM's recent history regarding advanced power train choices and availabilities, is that they will put those cost savings in their coffers and brag about to investors, and bypass giving their customers value.