"What would we all be using these days if the GUI as we know it was never invented"
That is a very general question. GUIs as we know it are a combination of:
Appropriate input devices
Bit-mapped displays
Computing power to multi-task
Common user interface guidelines
Application integration or communication
A graphical paradigm or metaphor (desktop, raw windows, bunch of beginner buttons or tiles, etc)
The lack of some of these could still give us software similar to what we are familiar with.
For example, if we were stuck with character cell displays, but had sufficient computing power you could easily have a text-based windowing systems. Applications could still have standard interface elements, multi task, share data, and even point-and-click with a mouse.
That reminds me, some video cards like my Video 7 card actually supported a graphical mouse cursor in text modes. (essentially using an arrow shaped "sprite" floating over the text)
On the other hand, if you still had bit mapped displays, but take away appropriate input devices, common user interface guidelines, application integration, and the common desktop metaphor then you are left with.... Windows 8