I don’t think GDevelop itself could send serial communications, however if your other hardware that can control those systems and you could get some sort of interaction based web-server on it (basically a private API), you can use the Network Communication Event Actions to send/receive data from that system.
I know that people have been using my MQTT extension (available in the built-in extension gallery) successfully to interact with internet-capable arduinos. It may be possible to make serial communication via WebUSB but that would be pretty hard to do.
Could you give me more informations about your MQTT extension and how it works please ?
Ultimately, if you or those people who have been able to interact with arduino could explain me how they did it, that would be great ! Thanks a lot !