Broadcom, on the other hand, should they have provided Linksys with a modified gcc, MAY have violated the GPL, but no one (but Linksys) can prove that.

The GPL is a one-step process from distributor to distributee. After Broadcom got their copy of gcc, they can alter it all they want. As long as they don't plan on distributing this new gcc, then they're fine with not releasing the source. And if they DO plan on distributing their gcc, they can also limit distribution as much or as little as possible. It's their prerogative as distributor. GPL doesn't say that you have to give your code to EVERYONE yourself.

However, once they distribute their gcc, it is distributed under GPL, and Linksys can re-distribute it freely. Broadcom cannot make Linksys sign any additional agreements which infringe on the rights the GPL provides.