evalin() is not the correct route, or at least not the preferred route. Just put a button on your GUI, view the callback, and paste the code from your m-file into the button's callback. That's what I'd do if the m-file is small enough.
If it's a really long complicated m-file, then I'd just make it a function that returns one of more variables that you can accept when you call it from your button's callback. Like in the callback
[out1, out2, out3] = my_m_file(in1, in2);
