Hello Simon,
To pass data between GUI's you should take a look at the following MATLAB Answers posts:
- https://www.mathworks.com/matlabcentral/answers/146215-pass-data-between-gui-s
- http://www.mathworks.com/matlabcentral/answers/95443-how-can-i-share-data-between-two-guis-in-matlab-8-0-r2012b
Lastly, "OutputFcn" is called after the opening function returns control and before control returns to the command line. For more information, check out the following documentation page: http://www.mathworks.com/help/matlab/creating_guis/files-generated-by-guide.html