I use goverlay to easily configure mangohud and then mangohud %command% on a per game basis in steam game launch options. Sometimes mangohud --dlsym %command% is needed for OpenGL games. Basically mangohud can only be used ‘without GUI’ since there is no GUI for mangohud. Goverlay is just a GUI for managing the mangohud configs and not running or launching mangohud.
Also there are keyboard shortcuts for launching and closing mangohud in game. Shift_R+F12 to toggle the hud on or off. Shift_L+F2 to toggle logging. Logs can be uploaded to FlightlessMango.com for some nice graphs.
Here are some commands to check if mangohud is properly installed:
mangohud glxgears
mangohud vkcube
Make sure to have lib32-mangohud also installed.
Maybe take a look at the gaming guide in the forum to make sure things are properly set up to run games in the first place: