It is easy to run a Grails 3 App directly from IntelliJ. Execute the main method in Application.groovy. Just click the green arrow.
Please, make sure you turn off bytecode verification to get runtime reloading
Run with Gradle BootRun Task
You can open a Terminal within IntelliJ and run the gradle task bootRun. Reloading agent will work using the Gradle wrapper.
Run with Grails Run-app
You can execute grails run-app directly from a terminal you open within IntelliJ.