In order to run your code with the name "gui", there needs to be a "gui.fig" file. That file needs to be a MATLAB binary .mat file internally. In MATLAB, .fig files are .mat files with particular variables stored in them.
MATLAB is looking at your .fig file and finding that it is not a .mat file internally.
Perhaps at some point you did something like imwrite() of an image on top of the .fig file.
You should use your operating system to have a look to see what it thinks the file is. On Mac and Linux you could
!file gui.m
to try to identify the file contents.
You will probably need to recreate the gui.fig file -- or restore it from backup.