Relatively little. While the gcc manual that comes with the distribution has some coverage of the C++ part of the compiler, it focuses mainly on the C compiler (though the information on the "back end" pertains to C++ as well). Still, there is useful information on the command line options and the #pragma interface and #pragma implementation directives in the manual, and there is a useful section on template instantiation in the 2.6 version. There is a Unix-style manual entry, "g++.1", in the gcc-2.x distribution; the information here is a subset of what is in the manual.
You can buy a nicely printed and bound copy of this manual from the FSF; see above for ordering information.
For versions 2.6.2 and later, the gcc/g++ distribution contains the
gcc manual in PostScript. Also,
Postscript versions of GNU documentation in U.S. letter format
are available by anonymous FTP from `ftp://primus.com/pub/gnu-ps'.
The same, in A4 format, are on
`ftp://liasun3.epfl.ch/pub/gnu/ps-doc'.
A draft of a document describing the g++ internals appears in the gcc distribution (called g++int.texi); it is incomplete but gives lots of information.