When playing with PPC port last autumn, me and @Yn0ga used command line like this:
$> qemu-system-ppc -L pc-bios -boot d -s -S -cpu G4 -m 1G -display sdl -drive file=haiku-boot-cd.iso,format=raw,media=cdrom -drive file=haiku-minimum.image,format=raw,media=disk -prom-env "auto-boot?=true" -device ati-vga,guest_hwcursor=true -serial mon:stdio
(which will block qemu waiting for gdb to continue)
and in another term :
$> powerpc-linux-gdb (note : gdb 14.2 recompiled for powerpc + python3 bindings + expat)
gef> gef-remote --qemu-user --qemu-binary /home/rj/src/haiku/generated.ppc/objects/haiku/ppc/debug_1/system/boot/openfirmware/boot_loader_openfirmware localhost 1234