A fresh created grails app contains the grails-app folder right beside the src folder. The grails-app folder contains all sources of the Grails application.
Why is this folder not inside the src folder?
Can it moved somehow?
No, it can not and should not be moved. The src folder is different than the grails-app folder for a reason. The grails-app folder contains the conventional Grails application structure where as src is intended to include other Groovy and Java sources that don't intentionally fall into the standard convention of a Grails application (e.g. TagLib, Domain class, Controller, Service, GSP, js/css asset, etc.)