Let's assume i have created a game, and then created ERC tokens (Example swords) so the player must buy a token to be able to play the game.
What if someone creates similar tokens in his wallet and injects them into my game as if they are bought from my tokens store
It can't be done, if you haven't set up your own rules in the game, meaning if someone tries to create a similar token in their wallet and then injects it into your game it will automatically exit the command in the rules you created.
For example issuing a verification command for an account that starts to log in and is suspected, otherwise the game you create will not be safe for people to play, logically the game maker itself cannot restrict people from trying to enter the game without the owner's permission.
How can people trust your game, every application that is created automatically must have its own security level and it is not easy for others to try to tamper with the application/game.
I think when you intend to make a game, the first step that needs to be prepared is the level of security, otherwise the game you make does not mean anything to people or people will not play the game.