For general advice and examples for how to create a GUI (and avoid using GUIDE), have look at this thread. Using GUIDE is a bad idea. It is outdated and forced you to keep using a narrow range of releases.
As for your problem itself: a GUI in Matlab is nothing special. It is just a wrapper around the functions you have already written. So you should first write the function that stores the data received through that serial connection. Then you can create the callback function that will call your data saving function.
