It is easy to run a Grails 3 App directly from IntelliJ. Just click the green arrow in Application.groovy
However, if you execute it without turning off bytecode verification the runtime reloading will not work.
Fortunately is easy to turn off the bytecode verifier. Edit the configuration which was generated once you run the app with the green button in Application.groovy
You need to add the -noverify to VM Options.
Turning off the bytecode verifier allows the reloading agent to work. This can have give you a great boost in productivity.
I picked this initially in a talk by Graeme Rocher