No you can't create a website using just CSS. In the past, styling in HTML became too complex and difficult to write and maintain. CSS evolved so it would separate presentation from content, making it less complex, redundant and overall more flexible. I believe CSS has always been about presentation and has nothing to do with functionality, otherwise it'd be coupling. Which should be avoided.
With HTML today, all you have to do is write the most basic plaintext website and present it with CSS. That method is recognized by the W3C today to be the best method for HTML pages.
In my opinion, design and interface has always been the same thing. At least in the context of usability, you are designing an interface. When you say GUI, you are talking about what is displayed to the user, not the back-end. So interface shouldn't be about how it works.
A better way of thinking about it is Model-View-Controller (MVC), a software engineering architectural pattern. The View is the user interface, what is displayed to the user. The Model is the the data and the rules applied to the data. The Controller talks to the Model based on what inputs are given.
MVC is implemented in alot of frameworks. Adobe Flex and Java Swing for GUI frameworks. Django and Ruby on Rails for web frameworks. etc. etc.