@Chris Miller You need to use the task

gskGenerateAccessors

to figure out what code you need for that plugin.

Many thanks, I think that points me in the right direction. It returns:

fun Project.`wsdl2javaExt`(configure: no.nils.wsdl2java.Wsdl2JavaPluginExtension.() -> Unit = {}) =
    extensions.getByName<no.nils.wsdl2java.Wsdl2JavaPluginExtension>("wsdl2javaExt").apply { configure() }

"wsdl2java" is a task so I (presumably) need to call it differently than I am (I'm new to GSK, Gradle and Kotlin so have a bit to learn)