I input the command into the terminal and it shows this:If you use complete path to the binary, it can be anywhere the executing user as access to.
First your gaudio_ctl command must work correctly from a terminal, under the user you want to use in your systemd unit. Only then focus on solving the systemd unit pecularities, it's the last step.
root@DietPi:~# gaudio_ctl -y "/root/camilladsp/camilladsp -o /root/camilladsp/camilladsp.log -p 1234 /root/camilladsp/camilladsp-48000.yml"
Ctl 'Playback Rate' not found, will not start playback exec