Sending data from a GUI to an Arduino nano via USB
Arduino Nano supports FTDI i.e. Virtual Serial Port. So all you have to do is initialize the serial communication using pySerial.

When the data is entered into the GUI, store it in some variables. Send the data in variables to Arduino using Serial Communication. You might have design your own communication protocol on top of Virtual Serial Port in order to identify the data.

You can also use the same library to send the acknowledgement or some data from Arduino to PC, if you want.

Hope this helps you.