Currently the script are written in Groovy (the Gradle Groovy DSL), because kotlin DSL was not a thing back then. But over time Kotlin script is getting the preferred option. The stronger type system improves significantly the developer experience, especially in IntelliJ IDEA.

https://docs.gradle.org/current/userguide/kotlin_dsl.html

This task might be doable incrementally, indeed the language is configuring Gradle via their own language DSL. But in the end it's the the Gradle model that gets configured and accessed. Which would allow to gradually convert the files.