Actually the MIDI in of the PBC will be going to the GT. The MIDI Output of the PBC will be going to the rest of your devices.
The ground connection is for shielding purposes. If you don't connect it on one side, your MIDI cable becomes a big noise antenna.
Pins 1 and 3 are used only on the bidirectional MIDI ports, for the opposite direction. All MIDI cables use 4 and 5 for normal communication. There's no conflict because one side is the input and the other side is an output. You need them to be on the same pins for communication to work.
The ground connection is for shielding purposes. If you don't connect it on one side, your MIDI cable becomes a big noise antenna.
Pins 1 and 3 are used only on the bidirectional MIDI ports, for the opposite direction. All MIDI cables use 4 and 5 for normal communication. There's no conflict because one side is the input and the other side is an output. You need them to be on the same pins for communication to work.