I wouldn’t count on the manager app to be there. I am not even sure if IntelliJ even installs the manager application by default. I’d rather try to get to your application. Do you even see compiled classes under build/classes/main now? If you don’t then you still have an issue with where you put your Java source files. Try to reach the context of your application.
Keep in mind, IntelliJ might handle the execution of your web application completely different from your Gradle setup. How important is it to you to get your web application running from the IDE?