While you can join any Grand Prix™ tier you have unlocked, we recommend the following to face a fairer competition and increase your chances of gaining Grand Prix™ rewards:

A minimum of 700 Team Score to play in Challenger
A minimum of 1000 Team Score to play in Contender
A minimum of 1500 Team Score to play in Champion 

If your best possible loadout Team Score is less than the recommended minimum for each tier, a warning will display.