@rjgmba hi and welcome to the forum!
https://www.mysensors.org/build/connect_radio decribes how to connect the radio for any node or gateway - it is not limited to serial.
A gateway has 1 transport and 1 controller interface.
The transport can be rfm69, nrf24, rs485, etc
The controller interface can be serial, ethernet(tcp), mqtt
So what you want is probably a gateway with rfm69 transport and mqtt controller interface.
Program the gateway based on the instructions on the first link you posted. Change the code from
// Enables and select radio type (if attached)
#define MY_RADIO_RF24
//#define MY_RADIO_RFM69
//#define MY_RADIO_RFM95
to
// Enables and select radio type (if attached)
//#define MY_RADIO_RF24
#define MY_RADIO_RFM69
//#define MY_RADIO_RFM95
connect the radio based on the instructions on the second link.
