What is a GUI installer?

It stands for Graphics User Interface(GUI), i.e. a windows installer.

Once upon a time, computer commands were entered a text. It was the days of DOS, not Windows. You typed commands next to a blinkiing cursor. You did not have all this point and click an icon and your program activates.

Very roughly speaking, A GUI installer attaches a Graphic (such as an icon) to the program so that all you have to do is point and click to make it work. So, on one hand, you can have a graphic, that is a picture, to work with or if the program does not have a GUI, then you probably have to use a command line. When you see a DOS Prompt, you are looking at a command line rather than a graphic interface.

Anything a human does to interface with the computer is graphical user interface (GUI)