Installing Gradle plugin 1.29 shows these messages in the log on Jenkins 2.146(ish), emphasis mine:

Oct 13, 2018 7:58:05 PM hudson.model.UpdateCenter$DownloadJob run
INFO: Starting the installation of Gradle on behalf of admin
Oct 13, 2018 7:58:05 PM hudson.model.UpdateCenter$UpdateCenterConfiguration download
INFO: Downloading Gradle
Oct 13, 2018 7:58:05 PM hudson.PluginManager dynamicLoad
INFO: Attempting to dynamic load /…/plugins/gradle.jpi
Oct 13, 2018 7:58:06 PM hudson.ClassicPluginStrategy createClassJarFromWebInfClasses
WARNING: Created /…/plugins/gradle/WEB-INF/lib/classes.jar; update plugin to a version created with a newer harness

Gradle plugin should update its toolchain to one supporting the newer classes.jar format. (Might be a problem with Gradle JPI Plugin, unsure).

CC daspilker