GP6 - Can I plug a MIDI instrument (keyboard or guitar) into my computer and see the notes I'm playing?

Yes it is possible. Guitar Pro includes a MIDI input that allows you to capture notes that way.

Please note the MIDI capture is not in real time though but one beat at a time. 

You will need a MIDI device for that purpose. Either a plain MIDI instrument such as the Jamstik guitar, You Rock guitar, MIDI Keyboard...Or you can also use a MIDI controller as an interface between your guitar and your computer.

Activate this option from the Sound > MIDI Capture menu and select your peripheral from the Device entry.