Section 0 of the GPL, with my emphasis:
The "Program", below, refers to any such program or work, and a "work based on the Program" means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another
Section 2b of the GPL, again with my emphasis:
b) You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.
And the viral nature of the GPL is cemented with this clause:
These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.
You might also want to check out the GPL faq, I'll quote a pretty relevant
part of it right here:
Q: If a library is released under the GPL (not the LGPL), does that mean that any program which uses it has to be under the GPL?
A: Yes, because the program as it is actually run includes the library.