task buildJavaFXPackager(group: "Build") << {
            enabled = COMPILE_FXPACKAGER
            copy {
                from "src/main/native/javafxpackager/shell"
                into "$buildDir/javafxpackager"
                fileMode = 0755
            }
        }

Because the task is defined as a closure, the "enabled" property will be ignored and treated as something to execute.