Lets assume you already have a folder which contains a Grails 3 app. For example you cloned a Grails 3 project from a git repository or you created an app from scratch with the grails create-app command.
Open IntelliJ and choose Open.
Choose the folder where the Grails 3 app is located.
Go ahead and tell IntelliJ to use the Gradle Wrapper. If you do so, the IDE will use the Gradle wrapper and you will not even need to have Gradle installed.