There are two licenses affecting GNU libstdc++: one for the code, and one for the documentation.
There is a license section in the FAQ regarding common questions. If you have more questions, ask the FSF or the gcc mailing list.
The source code is distributed under the GNU General Public License version 2, with the so-called “Runtime Exception” as follows (or see any header or implementation file):
As a special exception, you may use this file as part of a free software
library without restriction. Specifically, if other files instantiate
templates or use macros or inline functions from this file, or you compile
this file and link it with other files to produce an executable, this
file does not by itself cause the resulting executable to be covered by
the GNU General Public License. This exception does not however
invalidate any other reasons why the executable file might be covered by
the GNU General Public License.
Hopefully that text is self-explanatory. If it isn't, you need to speak to your lawyer, or the Free Software Foundation.
The documentation shipped with the library and made available over the web, excluding the pages generated from source comments, are copyrighted by the Free Software Foundation, and placed under the GNU Free Documentation License version 1.2. There are no Front-Cover Texts, no Back-Cover Texts, and no Invariant Sections.
For documentation generated by doxygen or other automated tools via processing source code comments and markup, the original source code license applies to the generated files. Thus, the doxygen documents are licensed GPL.
If you plan on making copies of the documentation, please let us know. We can probably offer suggestions.