I know you were looking for this earlier, here is some protocol information regarding the IIgs and 3.5" disks: https://archive.org/stream/Apple_IIgs_Firmware_Reference#page/n133/mode/2up

Corrections: https://archive.org/stream/IIgs_2523025_Firmware_Reference#page/n3/mode/2up

Raw disk access info: http://wiretap.area.com/Gopher/Library/Techdoc/Hardware/iwmiigs.txt

And finally, information on what pin-4 does on the IIgs floppy port: https://groups.google.com/d/msg/comp.sys.apple2/WH3jr4M97AM/S5a1XIBgKz8J

Another reference to the 3.5" enable pin: https://groups.google.com/d/msg/comp.sys.apple2/VFZJeum2Cy8/NTqWfXrsCvkJ