Leaving empty cast iron pot on a gas stove
I think you should be safe, but depending on the abrasive pad (plastic or metal?), you might have scratched the enamel.

Try the different methods to clean a burned enamel cast iron; most methods used boiling water with baking soda.

https://www.thekitchn.com/how-to-clean-burnt-stains-off-enameled-cookware-cleaning-lessons-from-the-kitchn-217457

or 

https://www.epicurious.com/expert-advice/how-to-clean-stained-enameled-cast-iron-pot-article