You can call functions from you GUI just like you normally would, but with one exception. You need to pass the handles as well, since that's where the GUI stores everything. So without getting too specific, here is how you get data from your GUI, to and external function, and back again:
% Lets say you have a var called 'data' that needs to be proccessed:
handles.data = data;
% And you have a function called 'some_function'
handles.output = some_function(handles.data)
% Now show it in, lets say a static text
set(handles.staticTextName, 'String,' handles.output);
% And to update the handles
guidata(gcbo, handles);
(And I just realised that I made an example where the entire handles thing werent nessesary, but it's a good way to do stuff, if you want to make sure where your data is.)
