Need cross gdb for device
You need to install the gdb-multiarch package, then run gdb-multiarch command to remotely debug your target.
I recommend getting gdb from the Linaro toolchain rather than the Ubuntu repositories. Download gcc-linaro-arm-linux-gnueabihf-4.7-2012.10-20121022_linux.tar.bz2 from here and you'll find:
bin/arm-linux-gnueabihf-gdbarm-linux-gnueabihf/debug-root/usr/bin/gdbserver
The latter is statically linked and of course built for ARM Linux.
Assuming you have networking already set up to your target board, copy gdbserver to it and run:
$ gdbserver --multi :2345
On your development machine, run:
$ arm-linux-gnueabihf-gdb
(gdb) target extended BOARD-IP-ADDR:2345
(gdb) set remote exec-file /bin/true
(gdb) run
and you're up and running with remote cross debugging.
Links to the Linaro bug tracker, mailing list and web forum are here. They're very active in ARM Linux toolchain, kernel, QEMU development as you can see from their release notes.