I recently wanted to learn GDB(Briefly). I searched in google with keyword " GDB tutorial"and google gave me this peter page. If you are writing program in linux it would be helpful to you. Peter started this tutorial with memory layout of a program and discussed gdb command with example. Any one who wants to know gdb could start from this tutorial. Anyway peter has also other good stuf . You can visit his site.
More inforation about GDB:
Official GDB User Manual
Norman Matloff tutorial on GDBPrinceton University GDB tutorial
RMS's gdb debugger tutorial
uchicago University tutorial