cmake-gui is CMake's bundled GUI wrapper for its command line interface. One can use this program for configure a CMake project and edit settings of such configuration. Use this tag for questions specifically about usage of the cmake-gui program.
From the reference documentation:
The cmake-gui executable is the CMake GUI. Project configuration settings may be specified interactively. Brief instructions are provided at the bottom of the window when the program is running.
Use this tag for questions specifically about usage of the cmake-gui program and not for questions about usage of CMake in general.