This is an older thread with rather low activity about a tool that doesn’t seem to work for very new boards according to chinobinos post from february.
So to get a constructive answer it might have been useful to provide some more details like tool- version used, system used, commandline used, type of board, type of ‘custom’ gb board, maybe even a screenshot of the process, maybe add link(s) to actual firmware and firmware that’s to be flashed?