On a tangential note, I also want to mention that I think the "glut" is something of a myth.
I say "something" of a myth because, while it's certainly true that there was a point where d20 products became less valuable than they had been previously, I don't think that was because of an overabundance of (bad) products.
The creation of the OGL was something that the RPG industry had never seen before. It essentially created a new opportunity for everyone by allowing cross-compatibility on an unprecedented (for the RPG industry) scale. By creating a new type of product that anyone could take advantage of, WotC basically expanded the market for publishers, and for consumers as well. This is called the "blue ocean strategy."
As with any successful use of that strategy, there was a flood of people rushing to both put out products for this new, wide-open category, and there was also a rush of people eager to purchase these new products. In other words, it was a bubble, rapidly expanding to accommodate everything that filled it. Bubbles, however, inevitably burst. Eventually the new market space ("d20 products") became more and more full, with an ever-widening pool of products competing for buyers' attention, along with purchasing slowing down as many of them bought enough products to be satisfied. Suddenly d20 products weren't the sure-fire moneymakers they had been previously.
The thing is, all of that is what's supposed to happen. In any industry, when a new field opens up, it's going to have a period of rapid expansion that'll pay off richly for the first people to deliver products in that area, followed by diversification among companies and a general slowing of growth (though if the growth rapidly stops, it can seem like the new market suddenly bombed). In other words, what happened with the d20 industry wasn't due to the OGL being "too open" or because too many "bad products" flooded the market; the market simply course-corrected after a while. d20 products were more valuable a few years ago than now because their value hadn't leveled off at that point, but now they have. In essence, current conditions are what the d20 market "should" look like.
There were other factors, of course. I honestly think that the 3.5 revision was the single biggest contributor to the bubble bursting. It lead to a lot of materials that were still on the market being perceived as being valueless (or at least, of less value than they were) and bringing down the worth of d20 products as a whole because of that. While it may have stimulated sales of new 3.5E materials, that wasn't nearly as great as they had been for new 3E materials three years previous; I think more was lost than was gained, overall, from 3.5E.
Another was the general decline of the U.S. economy. Depending on who you ask, the American market has been in decline for the last two or three years, and smaller companies are usually the first to feel that. While the smallest companies don't really have any overhead to worry about (the guy making PDF products in his living room probably has another job that pays the bills), ones that did felt the devaluing of the dollar most keenly. Combined with the bursting bubble for d20 products, and it was easy to see how a lot of publishers felt like the rug had just been pulled out from under them.
In short, the d20 industry hasn't failed, and I don't believe that the OGL (which is the basis for that industry) has either. It just enjoyed a meteoric rise followed by a very jarring return to earth. But while that landing may have been a hard one, it wasn't a crash, and I certainly think that the market could keep on existing as it does now for years to come.
The GSL, I think, doesn't really change that, since in function it's very similar to the OGL (a license that allows for a system-unified RPG industry). I expect it will have some effect, but it's too soon to tell exactly what it'll do.