If you would like to capture a log of everything that GDB displays during a debugging session you can do the following:
$ defaults write com.apple.Xcode PBXGDBDebuggerLogToFile YES
The statement above will turn on logging to a file. Now, you need to set the filename for the debugger output file. Use the following statement to accomplish that.
$ defaults write com.apple.Xcode PBXGDBDebuggerLogFileName /tmp/gdboutput.log
This will create a file in the /tmp directory called gdboutput.log, containing everything you see in the GDB command window during your debugging session.
Both of these commands need to be typed into a Terminal.app window. You only need to do this once, as your machine will remember the settings.