It's not under the GPL itself, but the Gnu project explicity lets you use parts of their license, as long as you call it something else.
The only part they reserve is the preabmle text, which you must get approval to use. That is the part that talks about the why of the liscense, and they don't want that part on licenses they don't consider free.
[link|http://www.gnu.org/licenses/gpl-faq.html#ModifyGPL|GNU FAQ]
I believe that liscenses and legal contracts in general are copyrighted material, you can't just take and copy somebody elses and the names changed. From what I have heard and seen though, that is actually a fairly common practice. Apparently so much is standard boilerplate that is cut and pasted that proving a specific contract is an outright copy is hard.
Jay