nixpkgs/pkgs/development/tools/build-managers/gradle/init-deps.gradle

gradle.projectsLoaded {
  rootProject.allprojects {
    task nixDownloadDeps {
      doLast {
        configurations.findAll{it.canBeResolved}.each{it.resolve()}
        buildscript.configurations.findAll{it.canBeResolved}.each{it.resolve()}
      }
    }
  }
}