does gradle support jpms for its own jars? if so, does anyone know what modules I'd need to use for a plugin written in java.

What do you mean with "support" and which jars? Gradle does not run from the module path, but from the classpath and plugins are dynamically loaded in custom class loaders, so the question seems to be a bit irrelevant. If you want to make your plugin a JPMS module for whatever reason, you probably have to configure the auto-generated Gradle API jar as dependency or something like that, but never tried something like that.