https://github.com/nigelsim/ppmadapter is an linux alternative to SmartPropoPlus.
Thanks for sharing, I will add that info to the manual.
You could also add https://www.instructables.com/id/RC-Transmitter-to-USB-Gamepad-Using-Arduino/ to the manual, as this is an USB Adapter that works for win, linux and mac as standard HID interface, meaning it does not need any driver. It basically allows you to connect any PPM outputting transmitter (which is a quite big class) as gamepad to your machine. Only thing is you have to build it yourself based on an Arduino - however you can see this as an advantage or a drawback, as you wish... ;) I like it and it for me the preferred way of connecting any transmitter.