It's obvious the GUI would have been invented. It didn't take too much knowledge to figure out the shape of letters could be sent to the video card dot by dot instead of as a single command and then interpreted. That part is the same as the switch from sending characters to printers and having them change the character to a series of dots.

I'm with you that the look might be different, but the basic concept would still be the same. It's not a real leap to say that having multiple programs running at once (both in task switching like I used with the DOS based DesqView, and multi-tasking modes like Windows 3.x and later versions of the Mac interface) would have been developed. Someone would have figured out you can't have true multi-tasking and paperless systems without being able to see multiple programs, so some sort of windowing system would have been developed.

The question is what the non-keyboard interfaces would have looked like if the mouse hadn't been invented first. Would we have seen a pen interface much sooner? Would everything have gone to touch screen, or pointer/screen (similar to the handheld devices of recent years)? As handy as a mouse can be, the coarse movements of a mouse might have actually slowed user interface development. More effort placed in a pen interface might have done us more good sooner.