You probably just need to do
source src/.gdbinit
first. See also "Configuring GDB" in etc/DEBUG.
Yes, I realized that.. a bit late.. I already killed that gdb session and started emacs rebuild.
While that was going on, I happened to read:
> It is important for the directory ‘src’ to be current so that GDB will read the
‘.gdbinit’ file in this directory.
Lesson learned: "gdb ./src/emacs" is not the same as "cd src; gdb ./emacs".