Create new device configuration in ESPHome for ESP32.
After initial configuration (you may skip connect/install steps), create file airbnk-gateway.h in config/esphome directory.
Paste contents of airbnk-gateway.h to created file.
In main device config yaml, make changes according to template. (Pay attention to TODO lines).
Install ESPHome firmware to ESP32 device and place device close to the lock.
That's it, enjoy. If you're here, i believe, you know what to do with ESPHome.
Get the code from repository, and flash it to ESP32 using PlatformIO or other IDE. Or, download only latest firmware_%VERSION%.bin file, and flash using ESP flasher or other flashing tool. Use https://github.com/formatBCE/Airbnk-MQTTOpenGateway/blob/main/partitions_singleapp.csv for partitioning reference.
Place chip near the lock and power it up. Make sure you use good power adapter!
Go find new WiFi network, named "AirbnkOpenGateway". Connect to it from your PC/phone, and go to http://192.168.4.1 in your browser.
Configure gateway according to your environment. Make sure you did type everything correctly before saving configuration. Use 2.4 WiFi network. Make sure you choose correct MAC address for lock, and MQTT topic corresponding to HA integration parameters.
After saving configuration, ESP32 will show you it's new IP address, restart, connect to WiFi and start sending telemetry and lock advertisements/commands to MQTT. Continue setup with https://github.com/rospogrigio/airbnk_mqtt
Although it doesn't really matter, you may want to make DHCP reservation for gateway on your router. If you'll want to re-configure gateway in the future, you can go to gateway web interface using it's IP in browser, and reset it from there. Also, gateway supports OTA updates over web interface, which can be useful.
If for some reason you don't have access to web-interface (e.g. WiFi configuration is incorrect, or changed, you may force-reset configuration of gateway by rebooting it 4 times with intervals less than 5 seconds in between. Basically, reboot it till blue LED stops blinking at start.
Cheers.