You can redeem a Roblox gift card by applying the balance to your account or by using it during the checkout process.

These cards cannot be used for Roblox Premium, however. These digital codes arrive almost immediately (within 5 minutes), so if you need a quick gift, you’re easily covered.

Thins is thanks to Roblox gift cards being digital codes (kinda like store vouchers) used on additional in-game content or character customization.