To open the GLS overlay in the editor or during gameplay, press and hold Alt + Shift + G.
To open the overlay on a mobile device, press and hold two fingers on an empty area of the screen.
To open the overlay on a console, press and hold the Gamepad Special Right button.
Configure necessary settings in Project Settings → Plugins → Game Logs System (GLS). You can set up key bindings to open the GLS log overlay during runtime.
To be able to view logs in shipping builds please read about it here.