ClassNotFoundException with main class, even though it is in the Jar file

Question

I have a personal project written in Java where I was using Maven so far. I'm migrating to Gradle to force myself to learn it (using it at work now). My intention is to produce a fat/uber jar.

When launching the .jar file, I'm getting a ClassNotFoundException because the main class cannot be found.

This is the build.gradle section where the Jar manifest is defined:

def mainClassName = 'GUI.APP_GUI'
jar {
    destinationDirectory = file(project.rootDir)
    duplicatesStrategy(DuplicatesStrategy.EXCLUDE)
    from {
        project.configurations.runtimeClasspath.collect {
            //println it
            it.isDirectory() ? it : zipTree(it)
        }
    }
    manifest {
        attributes(
            "Main-Class": "$mainClassName",
            "Manifest-Version": "1.0",
        )
    }
}

The Jar's manifest file does indicate the correct main class:

Manifest-Version: 1.0
Main-Class: GUI.APP_GUI 

Yet, I get an exception caused by:

java.lang.ClassNotFoundException: GUI.APP_GUI

What am I missing?

Thanks!