Pipeline step documentation is extracted from Jenkins plugins by the Pipeline steps documentation generator. That is run as a ci.jenkins.io job that generates two artifacts, allAscii.zip and declarative.zip. Those artifacts are downloaded by the jenkins.io site builder and are unpacked and inserted into the jenkins.io site when it is built.
I'm pleased that the documentation site generator works for a plugin that is built with Gradle. I thought that it was limited to plugins that are built with Apache Maven.
The documentation for the Pipeline step is available inside the Jenkins controller that has the Gradle plugin installed. However, it appears that the withGradle step from the Pipeline syntax snippet generator (as in /job/My-Pipeline-Job/pipeline-syntax/ ) does not have the same features as the withAnt step. The withAnt step shows a list of Ant versions that are configured as tools in Jenkins. the withGradle step does not show a list of Gradle versions that are configured in that Jenkins controller.
I assume that the withGradle documentation is more likely to appear on that Pipeline syntax page if the steps shows the tool versions inside the Pipeline syntax snippet generator.
I also see that there is no online help for the withGradle step while the withAnt step includes online help. The content of the online help is added to the Pipeline steps reference page.
In case it is useful, I created a 90 second video showing the Pipeline syntax snippet generator with the git plugin.
The documentation is https://plugins.jenkins.io/gradle/#plugin-content-capturing-build-scans-from-jenkins-pipeline. How can I make that show up on the page you linked?