Gateway operators are paid to successfully process data (which they can do if they stay online, have a fast connection to the Internet, and so on - not a shitty Raspberry Pi)

Gateway operators are paid to store data

Gateway operators are paid to mix client-side encrypted messages and shred message metadata

Gateway operators are staking their nodes to ensure cryptographically correct operation

If you want to see more, spend some time reading how blockchain-based networks with incentives work.