I don't want the GUI because my microSD size. I'm just happy with a command-line access. especially now that I downloaded some of the python packages I want to use. Now I only want the command-line access. I have Raspberry PI 3 Model B.
The "GUI" i.e. Xorg Display Server, PIXEL desktop environment, Openbox Window Manager and LightDM Login Manager actually takes little space.
Un-installing is possible, however, I suggest you just get rid of the space hogs; Wolfram, Mathematica, Scratch.