GRASS GIS 8 comes with both a wxPython-based GUI aka wxGUI (ui=wxpython) and command line text-based UI (ui=text).
All GRASS internal variables (see g.gisenv) are stored in the user's home directory in a hidden file called $HOME/.grass8/rc on Unix-based operating systems and %APPDATA%\GRASS8\rc on MS Windows. Note that these GRASS internal variables are not the shell environment variables and the rc file is not a classic UNIX run command file, it just contains persistent GRASS variables.
g.gui -d ui=text
Set default user interface setting to the graphical user interface (GUI) and launch the GUI:
g.gui -d ui=wxpython
Set default user interface setting to the graphical user interface (GUI) but do not launch the GUI:
g.gui -dn ui=wxpython
g.gui workspace=myproject.gxw