"Interest-bearing tokens from Green Planet"
gTokens are interest-bearing tokens that function as your receipt to be able to withdraw your deposited positions. So if you were to deposit 100 BUSD, you'd receive gBUSD. Over time, the value of your gToken should continue to rise as its value comes from the ratio between outstanding gTokens vs. the total amount of BUSD in the contract.
If you deposit assets in the planet vaults on the vaults page or if you supply any assets on Green Planet, you will receive gTokens from the protocol.