@erik I don't know. I am suspicious of the article since it shows a sketch that enables and enforces signing, but does not mention signing at all, so I don't know how many other assumptions are implicitly hidden in the article.
As far as I know, a atmega328p device has no wifi capability, so it has to use one of the other radios (nrf24, rfm69, etc) to communicate its data to a receiver. That receiver is designated gateway.
The only other way I can think of is running sensors directly on the gateway and have the gateway then communicate the sensor data using the gateway protocol in whatever transport the gateway has (which might be wifi).