By my reckoning, most SUVs are pretty awful, but their looks sell to people. That's why the Calibra sold. It wasn't a great car, but it did get bums in Opel seats and column inches. The point being, Opel need that now and they'll only get it by making cars like this concept.
Vauxhall/Opel certainly needs a car to get excited about.
Arguably, they need a competitive product range even more, and even sooner. The new Astra is such a product, so they're clearly on the right track, but the majority of their range isn't quite there yet and some of it is a long way from being there.
Much as I'd love to see the GT made though, the chances are slim. It'd be a good halo for the brand, but it's conceptually so far away from Opel/Vauxhall's bread and butter that building it as anything other than a loss-leader would be difficult, and there are very few companies, in the current economic climate, prepared to spend good resources on making something that wouldn't sell in large enough numbers to make even a sliver of profit.
It strikes me that Mazda and Fiat's relationship with the MX-5/124 might actually be the only reason the MX-5 is continuing into a fourth generation. Mazda is clever, but it's also a tiny, independent company by automobile industry standards, and while the MX-5 has always sold well it's also a relatively small global market to try and build a bespoke sports car for - engines aside, there's not exactly much crossover between the MX-5 and anything else Mazda sells.
Would it even exist were it not for the economies-of-scale benefits of also building the 124 in Hiroshima, and potentially spinning tens of thousands of extra sales off the platform? GM would be trying to do that effectively alone, and potentially without the benefit of US market sales, unless they dropped a bigger engine in there.