Gnome-Screenshot GUI: Which command launches that?

You may look for interactive mode

man gnome-screenshot

  -i, --interactive
          Interactively set options in a dialog.

Use:

gnome-screenshot -i

Anyway, to get command for a GUI launcher in menu, look for its .desktop file. Example:

more /usr/share/applications/gnome-screenshot.desktop

Or use MenuLibre tool

For other screenshot options without opening the GUI:

gnome-screenshot -a Grab a user selected area of the screen

gnome-screenshot -w Grab a window instead of the entire screen

gnome-screenshot -c Send screen grab directly to the clipboard

For additional options, in the terminal type your command -help

gnome-screenshot -help