Hard to say
I don't see Superman winning
In the context of a specific story, im sure the writers would eventually give Superman the nod over grail. The same way, they give Superman the win or tie against Darkseid. However, based on her showing, I don't think Superman can beat her. Her weapon was able to pierce Shazam, it would probably have the same outcomes. Furthermore, his heat vision may be able to stop hers.
All in all, as much as I want Superman to win, I think she was portrayed stronger. I will hold my breath for Doomsday clock because Superman will face off Dr. Manhattan. After that, I'm sure he will have some BS excuse for me to give the nod to Superman over Grail