user interface vs GUI

user interface and GUI both are nouns.

Word	Noun	Adjective	Verb	Adverb
user interface	Yes	No	No	No
GUI	Yes	No	No	No

As nouns, GUI is a hyponym of user interface; that is, GUI is a word with a more specific, narrower meaning than user interface:

user interface: (computer science) a program that controls a display for the user (usually on a computer monitor) and that allows the user to interact with the system
GUI: a user interface based on graphics (icons and pictures and menus) instead of text; uses a mouse as well as a keyboard as an input device

Other hyponyms of user interface include CLI, command line interface, graphical user interface.

user interface (noun)	GUI (noun)
(computer science) a program that controls a display for the user (usually on a computer monitor) and that allows the user to interact with the system	a user interface based on graphics (icons and pictures and menus) instead of text; uses a mouse as well as a keyboard as an input device