I have all three files in a single folder, and I opened the main program with the GUI. When I ran it, it gave me an error saying that it cannot find the first function called. I've got Octave 6.1.0 if that is significant. Have I missed something?
your working directory (where Octave is looking right now) is probably different than where you saved the files. Octave can't know where everything is on your computer. In the upper left of the GUI is a folder tree that will show your current location and let you navigate somewhere else. Use that (or the command line if you know how), to change your current working folder to the folder where you saved those files. You will be able to see the files in that folder tree in the gui when you are there. then, try running the files and let us know what happens.