When you are using the guide it is actually a gui for making gui's. What you are actually doing is making an m-file and this m-file makes a figure with some buttons, bullets and values. These are the callbacks and they are nothing else than changing values in the m-file's workspace.
For instance type
and you see Matlab's sound capability gui. If you would instead type
you get to see the m-file which generates this figure and its callbacks. In this case the volume bar is an example of a slider.
It appears that you only know how to make gui's with guide. Is that correct? I never made one with guide, but there should be a way to call other functions. If you cannot find it, I would suggest to add some arbitrary callback, save your gui and then change it in the m-file to execute another m-file of your choosing.