I had one of those Gulbransen's in my living room, many years ago along with the Isomonic Leslie's,
I found the biggest problem with them, (I was a Gulbransen repair tech.) was the rotary Contact devices (Mercotac's) that were utilized to pass the amplifier outputs, to the Rotating 6" x 9" Speakers that were mounted on the rotating drums. They used Liquid Mercury as one of the rotary Contacts, and a ball bearing assembly as the second speaker wire.
Great idea,,,,, "Except the mercury oxidized, and they then became very intermittent" ,
By now I doubt that any of the Mercotac's that were made back then, are working at all !
Shaking them like a thermometer would fix them momentarily,
but replacement was usually necessary !
I personally replaced dozens of these devices !
I remember buying a carton full of these devices, from their manufacturer a few times,
and they were not cheap.
Allen's Gyro-phonic, speakers went an expensive step beyond,
by using Commutator Rings with Contact Brushes to feed audio through to their rotating speakers,
to (almost achieve), Leslie's rotating effects !
Hauptwerk's audio settings easily [simulate] the same effect, (C-C# Split), If not improve on it,
Especially as far as 100 % Reliability is concerned !
The Speaker cabinets for the Hammond X66 an X77, produced a somewhat similar effect,
but they achieve their effect", by using motorized, Rotating Variable Capacitors,
to provide what they called their, (Space Expander- Iso-Monic ?), and also Vibrato effects !
(I do love my Hauptwerk Organ, Thank You HW.) Thank you for your Replies !
Mel