Not necessarily. If your startup is based on selling the software itself, then the GPL might be detrimental to your business plan. (Then again, it might not.)
But not every successful business plan involves selling the software itself. There are countless web services that could GPL their software and keep right on making money. Some have, in fact. The code that runs Slashdot is released under the GPL, but it takes more than code to make a successful online community.
The software that runs on FreeHand digital music notebooks (http://www.freehandsystems.com) is released under the GPL, but the major selling point isn't just the software; it's the specialized hardware.
