Turtorial
Go through the Links. [2] will be a good start point, and [8] may be the best turtorial I've ever found.

Common commands:
b foo.c:123
disp foo
save-tracepoints
source
shell vi
wh : Set the height of a specified window.
l/l - : list source code forward and back
gcore : produce a core dump

Moving around:
jump out of loops:
u
jump out of current function:
fini

Links:
1. Guide to Faster, Less Frustrating Debugging
2. RMS's gdb Debugger Tutorial
3. Debugging Under Unix: gdb Tutorial
4. Mastering Linux debugging techniques
5. Debugging with gdb: The gnu Source-Level Debugger
6. A Tutorial on the Intermediate Use of GDB
7. GNU Project Debugger: More fun with GDB
8. Using GNU's GDB Debugger