Within your Game Mode, write an enum to store whether the game is currently being played, won, or lost, as well as functions to handle the playing state.

Check out the source here, with the project available for download:
(00:05) - Intro
(00:25) - Creating a Custom Enum
(02:21) - Setting up the Variables
(05:24) - Game State Setup
(09:05) - Displaying the Game State in the HUD
(12:43) - Testing