I highly recommend reading the GPL FAQ. Understand that there are different flavors of the GPL, and which one the libraries you want to use will affect your project differently. This page describes the different GNU licenses available. I believe the article on "why to not use the LGPL" is rather telling.
Understand that there is the doctrine of GPL, and what can be practiced with GPL. To walk out the intricacies of these libraries, how it will affect your commercial project, and what you can do to work around those problems--talk to a lawyer. There are lawyers who specialize in GPL counseling. The GPL is constrained by local laws which may supersede the terms of the license. Richard Stallman (Mr. GPL to you) has an agenda to make as much software as he can "free as in freedom, not as in beer".
The key terms to understand are: publication, linking vs. embedding (LGPL concern), and distribution. The GPL variants require that you make your source code available to anyone who requests it if your project falls within the purview of license. It does not require that you host it on a web site where anyone can download it. Making it available can be a snail mail request, and sent via paper print out if that's the way you elect to do it. While that's not in the spirit of GPL, it fits in the letter of the license.
If you are required to make your source code available, most companies will not consider it because in their eyes if someone else has access to their "secret sauce" then the barriers for competitors to release a new version of their software will be nearly eliminated. Even if you don't distribute your build scripts, there is nothing that says someone else can't come along and create them on the distributed copy of the source code.
NOTE: It may be worth checking to see if the library you want to use has a BSD/MIT/ASL style licensed version out there. In many cases there is, which side-steps the whole issue with GPL. The BSD/MIT/ASL licenses are designed to allow companies to incorporate the library into their commercial projects without any viral side effects. While there is nothing requiring the users of the library to contribute back, in most cases they do anyway. In short, this shows that some of Richard Stallman's arguments are flawed. RS does provide a purpose, as the extreme needed to provide alternatives where a happy balance where should be. Not all GPL supporters agree with RS' interpretation of his own license which also adds to the confusion.
This faq entry answers the question most succinctly.