This gem just wraps Xlib's XInput2 extension and makes its interface available in ruby through ffi. XInput2 documentation can be found in its manual (functions starting with XI belong to XInput Version 2 as defined in the header). No functions have been renamed or abstracted away in some objects. It's just the plain XInput2 API.
For a lightweight wrapper and a more ruby style access to X11 have a look at ruby-xlib-objects'.
gem install xlib-xinput2
All constants and methods of XInput can found under their original name in the Xlib namespace.
d = Xlib::XOpenDisplay(':0')
maj = FFI::MemoryPointer.new :int
min = FFI::MemoryPointer.new :int
maj.write_int 2
min.write_int 1
Xlib::XIQueryVersion(d, maj, min)