There is a difference between Usability Testing and GUI(Graphical User Interface)Testing.
Usability Testing: It means Testing the application whether it is usable or not, whether it is giving proper error messages or not if we operate this in an another way(other than it's behavior). Let us assume that , The end user has no computer knowledge then we have to test from end user point of view.Before going to do Usability testing it will be better if we know who are going to use this application. But it may not possible in all situations.
In short Usability testing means , verifying that how easily the end user can use the application.We have to test this in such a way to make the application easy to use.
GUI Testing:This is verifying the User interface of the application means how the front end design, font color, Text format, Labels,background,foregroung Etc. This is the process of verifying the look and feel environment of the application.
Both testing are very important for web applications.