Functions in your GUIDE program can call other m-files no problem. In some function in your guide m-file, at the place where you want to call this other function, just put its name on a line of code and pass it whatever input arguments it needs and accept whatever output arguments it returns to you (if you want or need them). For example, inside a button callback function in your GUIDE program:
[out1, out2] = MyOtherFunction(in1, in2, in3);
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