The most likely case is that they just don’t want to deal with support requests for alternative uses. In all likelihood they’ve got a thinly customized build of the Semtech forwarder or known derivative taking the standard configuration files like global_conf.json
But to start over from upstream sources probably all you really need to do is to figure out what pin and polarity they are using for reset. If you can get a root shell on the pi you can likely find and examine their scripts, or maybe they leave the gpio exported.
So, I’ll try to install the Gateway OS in an SD card e hope that it will work (I know it is risky, but right now that is the only thing I can do).
It’s not all that risky if you install it on another card.
But it’s also not your only choice: if it works, great. But all the various DIY guidance for using random concentrator cards on a pi for TTN etc should be applicable in the basics - ie, look up stuff on iC880A, RAK831, RAK833 in SPI mode, grafting a multitech card onto a pi, etc.