Gdb comes with extensive online documentation. Much of the
doucmentation is available within gdb itself through the help
command. J2c defines a couple useful commands that are documented
through help. help user-defined will list them, and
help cmd will display help for a particular command.
While gdb's built-in help can be useful, the reference manual contains
more complete documentation, including a breif tutorial. The location
and format of the reference manual varies between platforms.
On Macs, gdb's online reference manual is available through
<file://localhost/Developer/Documentation/DeveloperTools/gdb/gdb/gdb_toc.html>.
The gdb source distribution also includes a double-sided quick reference
card, which Apple has been thoughtful enough to package as a .pdf
in /Developer/Documentation/DeveloperTools/gdb/refcard.pdf.
Gdb's online reference manual is available on linux boxes
through emacs and the standalone info command. In emacs, type
<ctrl>-h i, search for gdb, and follow the link by either
middle-clicking on the bold text, or hitting return while the cursor
is on bold text. Redhat does not seem to install the quick reference
card anywhere, but the Apple version is available at
/p/sss/project/apple-gdb-refcard.pdf.