The GameID is a unique number assigned to a game to help with troubleshooting and technical support requests. GameID will come in handy when you are reporting an issue or need help with the game.
GameID can be viewed in the game Loading screen and Settings screen of the game. Just tap on the GameID to copy and then you can paste it to your message or mail.
If the game is loading fine you can find GameID at the bottom of the Settings screen. If you are having trouble loading the game then you can get the GameID shown at the top left corner of the Loading screen. Sharing GameID helps the support team check your game and address the query faster.