When I purchased a TC Helicon GT I learned the unit would only work with a Guitar. I am not talking about using scales for harmony.
There is a TC Helicon, I think it is “M” and you are able to connect a Midi wire from your keyboard to the unit which will control the notes in your harmony. (Voice)

Since I play keyboard and guitar I decided to return the “M” model and buy a TC Helicon touch live -- which gave me the ability to use Guitar or keyboard. It was additional $250 but I received a discount of $100 at Guitar Center bringing my cost to $400 a difference of $150 -- well worth the difference. (for me)
For the difference in price I have a unit that can record Loops to go along with my voice, a Touch screen and some areas which made the unit a keeper. (For me, I hate to say, “I should have”)

John C.