Here's how to get the gem in Rocket Pass:
1. Reach the required Tier: Each Rocket Pass has a specific tier where the gem item is unlocked. The tier number varies from season to season.
2. Progress your Rocket Pass: You gain Rocket Pass XP by playing matches, completing challenges, and by getting Rocket Pass boosts.
3. Unlock the Tier: Once you reach the required tier, you'll automatically unlock the gem and all the other items associated with that tier.
Important Note: There's no specific "gem" item in Rocket Pass. It's likely you're referring to a specific item that is a gem-like color or design. You'll need to check the current Rocket Pass to see what item at which tier you're looking for.
Let me know if you have any other questions about Rocket Pass, or Rocket League in general!