Yes, using WP in general is not a war crime, but that’s because it’s usually used for illumination in large open fields, not in densely-populated urban centers.
Using WP over Gaza is. There is no way to avoid civilian casualties when using WP over Gaza because of how densely populated it is. There is no way to know who will suffer, how many, or for how long. All we know is that they definitely won’t get the medical treatment they need because Netanyahu’s government has been preventing humanitarian aid from coming in, prevented them from having access to food or clean water, and shut down their access to electricity.