One is game when one is interested in and excited about doing the thing. Example: "Do you want to see a movie?" Reply: "I'm game."

I'm game says that you want to do it. Example: "John, we're going to play cards. Do you want to join?" Reply: "I'm game." Another example: "Would you like to go out tonight?" Reply: "I'm game." Another example: "Do you want to play pool?" Reply: "Sure. I'm game." Another example: "Have you asked Jim? He's usually game."