Are you talking about querying for all current games? There is no direct way to do this. You can get all streams and grab the game from that object.
You can see top games with the games API: https://dev.twitch.tv/docs/api/v3/games/ You can search games with the search API: https://dev.twitch.tv/docs/api/v3/search/
What are you trying to accomplish? Interested in your use case. If you’re looking to store this data, please make sure you’ve read over the developer agreement about data storage: https://www.twitch.tv/p/developer-agreement