RG will grab your song, read the RG tag on the file and apply the volume difference on top of your current volume setting.
Real life example: you're listening to a song that has a RG tag written with: -6.31 dB (track gain) and -7.04 dB (album gain) - the same example i told you above.
You turn on the RG feature on your X1 in track gain mode, for example, and play the song. Let's say you have the volume set at 20.
Your X1 will reproduce the song at volume 20, but will apply a volume difference of -6.31 dB (in this case it will turn down the volume a little bit).
If all songs in your library have an RG tag written, you'll hear them at the same volume because the differences applied depend on the track and the objective is to eliminate volume differences between the tracks. Every track will have a different gain value in order to achieve the same volume altogether.
Still unclear?