It's not a GDB built-in, it's a command defined in emacs/src/.gdbinit.
That's why I told you to start Emacs from .../emacs/src.
Alternatively, you can "source .../emacs/src/.gdbinit" and then
use xbacktrace.
Ah, that makes sense! I probably should have looked up some documentation on
debugging emacs, but I was a bit busy that night.
Unfortunately, after my conversation with Eli I (foolishly) decided that I no longer
needed that emacs process so it is not available to play with anymore.
If it happens again though, I will post the xbacktrace here.