Game time means that it’s time to go what has to be done.
Time to play/work/accomplish what needs to be done.
The time at which an activity is about to begin.
1. Ok team, it’s game time! Let’s put our energy into having fun and winning this tournament!
2. When are we going? I’m not sure but let’s make it a game time decision.
3. George, it’s game time for you. I want you to land this multimillion dollar deal.
4. Kids let’s go! It’s game time!
beginning, commencement, get-go, kickoff, start, starting time