The "Kings" of the Forest of the Greats (Maxwell, Cyrus, Ivan, Chaplin...) are not the same as the firstborn returners Alex, Solomon and Ramesses (who are confusingly the reincarnations of famous kings).
I'd say Gale is definitely on par, and probably stronger than most Kings. I can't see Ivan or Eric standing up to Touya, Ein, Newton and Picasso on their own. In a straight up fight, I wonder how Cyrus would fare against her.
But yeah, she's nowhere near the power of the firstborn returners. Alex simply destroyed her Eternal Darkness with no effort, a plan she had prepared for for years.