To be honest, it was a lot more than the gsl that drove away third party support. Fred hicks did a nice little postmortem when shutting down his 4e line, one bad egg, which he and some other decent devs were doing.
Notable was the online character builder, but what was really interesting is that he found that 4e was much harder to design for- I wrote up an entire 4e sourcebook a couple of years ago, and I agree with him.
Frankly, one of the upsides of designing for 3e is that 3e is a pretty crappy, unbalanced game and that makes it easy to design for. Pathfinder's new classes display this pretty clearly- they're a mixed bag, some overpowered and dominating, others so pointless you wonder why they bothered. But this is ok in 3e, because 3e is like that, it's an undemanding, hobbyists sort of design. Some of it's fans call this a bonus but, let's be frank, it's not.
Compare that to 4e. A bad mechanic in 4e sticks out like a sore thumb, wether it's something a 3pp comes up with, or something wotc comes up with. People playing 4e very quickly came to understand it's problems, because it's a very lucid, coherent design- from overpowered or underpowered powers, to entire classes, to systems like skill challenges, 4e asks more of it's desginers because it's a better core design.
I suspect even the modules, apart from the key errors made in them (too many fights, ect), are being judged more harshly in part because the system works better, and so is more obviously broken when it doesn't work.
There have even been 3pp providers who have fallen foul of this, people who put huge time and effort into major projects, only to find a lot of people calling them out for what they saw, with some good reason, as poor desgin. I'm not trying to start anything with anyone by saying that, but it has happened.
The reason i'm talking about this isn't just to toot 4e's horn, it's a clear part of why 3pp is harder for it- it has higher barriers for entry for anyone trying to design for 4e, for real.
All factors taken together- even if 3pp had access to the cbuilder, and other forms of support- i'm just not sure that anyone could expect to match the products that we saw with 3e, in part because making those products properly, is harder for 4e, and fans of 4e are more demanding of the system.