There are two places where gas is paid: 1) when the customer pays to these intermediate addresses 2) when the funds are moved from the intermediate to the final address

1) is always paid by the customer that initiates the payment. 2) can be paid by anyone, as these contracts are permissionless and anyone can interact with them. At the moment Coinbase picks up the tab for doing this, but we will likely not be doing this in the long-term