Image you want a sensor somewhere deep down. Deep down in your basement, in your cistern or your little summer house in your garden.
Lucky you: Someone put a cable there. Not a 230V/115V type one (anymore). But something for a doorbell maybe, two spare wires on another low voltage cable or very old wires you would not dare to put 230V onto anymore. So you can have power there. 12V/24V or so. But still you don't get any data up from there.
Or another case: There is power and there is some data line (analog audio, FBAS video, or so). But not free wires for your data. Anything which is not disturbed by a 5.5 or 6.5 MHz signal and does not use on of these these freqencies is fine. You can simply add the gateway's signal. Examples might be the lines of your doorbell-switch. Or a line for a CCTV camera, or your ananlogue door camera with it's differential video line.
This is where this gateway kicks in: It exposes a ESP32 http, https and mqtt API via a little modem. Not with megabits data transfer, but with 19.2 kBits. I'm using the Yamar SIG60 UART over Powerline chip for that. It can put a Master-Slave bus on almost any line via PowerLine-like communication.
The SIG60 is used as a UART Bridge, we are not using LIN here (at the moment). The master can wake the slaves (and vice versa) and keep them awake using a cyclic message using the SIG60 sleep mode management. The same keepalive message is a round-robin token which allows the slave(s) to send as well (master-slave-polling). A slave can send a request to the master if it has the token (valid 250ms). The Master will then try to execute the request and send the response back to the slave. During working on the request the token stays at this single slave (but is repeated as keepalive).
I retrofit mode there is no polling/token: Anything any slave sends is simply published to a pre-configured mqtt/http target. No feedback. Only one tiny restriction: No "0" bytes allowed as theses are interpreted by the SIG60 devices.
On the slave(s) side there has to be the same modem an a bit of logic of course, besides from that anything which has an UART interface supporting 19.2 kBit/s is fine.