How to run java Google App Engine locally with app-gradle-plugin for yaml?

TL;DR appengineRun is only available for appengine-web.xml based projects. If you want to use app.yaml, you must provide your own server, for example Spring Boot with Jetty or Tomcat.

To run your application locally, you must provide your own server.

This guide shows how to test your application using app.yaml alongside with the app-gradle-plugin, on section Testing your application with the development server:

During the development phase, you can run and test your application at any time in the development server by invoking Gradle:

gradle jettyRun

Alternatively, you can run Gradle without installing it by using the Gradle wrapper.

As said on this comment on GitHub:

If you want to use app.yaml from your root directory, you must upgrade to Java 11. Learn more here. With the Java 11 runtime, you must provide your own server, for example Spring Boot with Jetty or Tomcat. The appengine:run goal does not work for app.yaml based projects because each server has a different start up command i.e. spring-boot:run for Spring Boot.