Thank you, I really appreciate your help, but it still doesn’t work. 
I checked the files, and they are there, maybe the problem is that it is looking for the files at:
C:/workspace/offline/build/offline-repo/org/grails/grails-gradle-plugin/3.1.4/grails-gradle-plugin-3.1.4.pom
but they are actually at:
C:/workspace/offline/build/offline-repo/org.grails/grails-gradle-plugin\3.1.4/grails-gradle-plugin-3.1.4.pom
I don’t really know if it could be the problem or not.
Also if I enable the online repositories, gradle says
configurations = [configurations.testRuntime, buildscript.configurations.classpath]
line is problematic: Cannot get property ‘testRuntime’ on null object