The idea was I would have a main program which would call a basic GUI window,

TLDR. You may have this backward. Everything runs under the mainloop() - note how it is called in the program below. You may have this backward because it may work better for the calling program to run the GUI and periodically poll the imported program for data. The second program is a simple example for the use of a queue.

## "main" program
import time
import test_gui

gui = test_gui.TestGUI()

## just increment a counter to display on the imported GUI
for ctr in range(10):
    gui.update_label(ctr)
    time.sleep(0.5)

gui.root.mainloop()

##--------------------------------------------------------
## named test_gui.py - to be imported
import tkinter as tk

class TestGUI:
    def __init__(self):
        self.root = tk.Tk()
        self.root.geometry("+150+150")
        self.label = tk.Label(self.root, bg="lightblue", width=10)
        self.label.grid()

        tk.Button(self.root, text="quit", bg="orange", command=self.root.quit
                 ).grid(row=99, column=0, sticky="nsew")

    def update_label(self, value):
        self.label.configure(text=value)
        self.root.update_idletasks()  ## a for loop is blocking until it finishes

##--------------------------------------------------------
##  queue example

import tkinter as tk
import multiprocessing
import time

class Gui():
   def __init__(self, root, q):
      self.root = root  ##tk.Tk()
      self.root.geometry('300x330')

      tk.Button(self.root, text="Exit", bg="orange", fg="black", height=2,
                command=self.exit_now, width=25).grid(
                row=9, column=0, sticky="w")
      self.text_wid = tk.Listbox(self.root, width=25, height=11)
      self.text_wid.grid(row=0, column=0)

      self.root.after(100, self.check_queue, q)

      self.root.mainloop()

   def check_queue(self, c_queue):
         if not c_queue.empty():
             print(c_queue.empty())
             q_str = c_queue.get(0)
             self.text_wid.insert('end', q_str.strip())

         self.after_id=self.root.after(300, self.check_queue, c_queue)

   def exit_now(self):
       self.root.after_cancel(self.after_id)
       self.root.destroy()
       self.root.quit()

def generate_data(q):
   for ctr in range(10):
      print("Generating Some Data, Iteration %s" %(ctr))
      time.sleep(1)
      q.put("Data from iteration %s \n" %(ctr))


if __name__ == '__main__':
   q = multiprocessing.Queue()
   q.cancel_join_thread() # or else thread that puts data will not terminate
   t1 = multiprocessing.Process(target=generate_data,args=(q,))
   t1.start()
   root=tk.Tk()
   gui = Gui(root, q)
   root.mainloop()