As Koraks said- this makes the solution way more complex than it needs to be. You don't need ESP-Now at all. But your block diagram would work since ESP NOW allows for a many to one protocol.
But, you don't need the "Send to MQTT" device. The "Receive ESP-NOW" device can publish the MQTT message itself.
What is a "wifi module"?
If I were designing this for a real-world system, I would eliminate the gateway altogether and just have each sensor ESP publish the MQTT message. But that doesn't seem to meet the parameters of the assignment.