my ESP32-POE just arrived and I started playing with it, but currently I am not sure which board I shall use to create a new project in PlatformIO. I used "ESP32-GATEWAY" and was able to compile and run the code example for this board, but I am not sure if this is the correct way to start. What do you recommend to use for development?
I don't think you would encounter any major problems. You might have problems with the Ethernet because the ESP32-GATEWAY and ESP32-POE have different clock sources. The GATEWAY uses external clock source and the POE uses the internal clock. The board is simple enough and you can try the configuration for other similar boards.
You can also try the ESP-IDF or the Arduino for ESP32. We have provided Ethernet examples for those two environments: