I am MeFisto94 
I use the following “flow”
./gradlew buildSdk # (or use ./gradlew assemble)
netbeans/bin/netbeans # (.exe on Windows)
Then I open the Project “sdk” (which is actually a Project-Group) and right-click “Create MacOs Application”, so I can launch it without being run through another NB instance.
Alternatively you should be able to right-click on “sdk” and select debug/run from there.
Note: Technically it shouldn’t matter if you build the SDK through gradlew or the opened netbeans, however calling gradlew ensures that the Engine itself is built, which is something I don’t know if the sdk does.
Note: Ensure to delete your netbeans folder before running the new gradlew for the first time, or else you’d download netbeans 8.0 and the build will fail