If a unit you want to use is unavailable during Galactic War, that means one of two things.
That character hasn’t been unlocked yet.
The character was downed during one of your Galactic War battles.
Remember, the health and status of your Units will be maintained until you reset your Galactic War.
If you’re unable to complete Galactic War with your leftover units, reset! This will heal and revive all your units, but will also set you back to the beginning of your Galactic War.