Looking to do this in the opposite direction

[Schkrapnel]

Hey there,
I'm currently looking at writing my own program for an Arduino of some kind (haven't decided just yet) that would be able to emulate a gyro input using a mouse and pass that into a Switch by emulating a Bluetooth controller (sort of like how this repo does it). The thing is, I'm not really sure what the gyro input signals are supposed to look like or where you're reading them from. I tried looking through the source code to find where the datastream could be but I am very, very stupid. Would anyone mind helping me out? (Also yes, I am planning to try playing Splatoon with this LOL)
Thank you so much in advance.