The avr-debugger library basically opens a GDB server that responds to serial commands and is thus usable by avr-gdb. As a drawback, you must not ever use Serial in your own program, because you would be conflicting with the GDB logic of course.
Can you reproduce the minimal debug-a-blinky-sketch example for the Uno at the bottom of How to debug on Arduino mega 2560 - #2 by maxgerhardt?