First of all, the option to redeem a Roblox gift card cannot be done within the game. You have to do it on the official Roblox Game Card Redemption Page.
You will now need to convert the Roblox credit into Robux so that the currency can be used within the game. Miraculously, the link to convert Roblox credit into Robux isn't listed or linked to anywhere on the Roblox Game Card Redemption Page, so it's no wonder that the whole process to convert a Roblox gift card into Robux is overly complicated ...