Maybe

build.gradle

project(":gwt") {
    apply plugin: "gwt"

    dependencies {
        compile project(":core")
        compile "com.badlogicgames.gdx:gdx-backend-gwt:$gdxVersion"
        compile "com.badlogicgames.gdx:gdx:$gdxVersion:sources"
        compile "com.badlogicgames.gdx:gdx-backend-gwt:$gdxVersion:sources"
    }

    gwt {
        gwtVersion='2.5.1'
        modules '%PACKAGE%.GdxDefinition'
        src += files(project(':core').sourceSets.main.allJava.srcDirs) + files(project(':core').sourceSets.main.output.resourcesDir)
        compiler {
            strict = true;
            enableClosureCompiler = true;
            disableClassMetadata = true;
            disableCastChecking = true;
        }
    }

}