If you're on Windows you could use the atbackend.exe from Atmel Studio 7. It has a gdb-proxy mode which you can use with avr-gdb.
i.e. "atbackend.exe /avr8-gdb-ports=2345"
see
here for instructions on Microchip web site.
AFAIK, fpc doesn't have support for this MCU family ( 0-series a.k.a. avrxmega3 ). But Christo has an utility which translates the ADTF files and was kind enough to send me the translated files for this family to see if I can work with them. For me those units works ! ( mind you that I didn't test the board, I mostly use bare IC's on breakout boards on breadboards. I'm just a hobbyist ).
Also I use avr-gdb 8.3 ( patched also from Christo ) and cross compiled to windows from Michael Ring .
All and all, I use mcu -> m/n/edbg -> atbackend.exe -> avr-gdb -> Lazarus (remote gdbmidebugger) which works ( with some hiccups here and there ) and I can debug all the avr mcu that I use.
EDIT: Apologies ... I missed the "I am using a Mac ..." . You could use a windows vm though ...
EDIT2: I missed also the title " .. non-windows platforms" ... I need a break ...