Does the neutral light come on when in neutral?
The problem almost has to be a problem with the diode assembly, the wiring between the diode assembly and the neutral switch or the neutral switch itself.
Hint: You would get more replies if you post the issue in the threads name. Like "GL1500 only starts with clutch."
Welcome anyway and you can post you year and model so all can see as part of your signature or avatar..
I presume all the parts replaced were good? What year is this machine? How long have you owned the bike and when did reverse last work? What other work has been done on it? Does the R light illuminate? Does the oil pressure light work as intended? Does the side stand light work? And as Mike asked earlier does the neutral light work?