Mp3Gain adds/uses the Relative Volume Adjustment frame in the mp3 file (supposedly), which is a native id3v2 frame, to kind of normalize the tracks to a similar level so that you shouldn't have to readjust the player volume control for every track. It's not supposed to alter the actual audio data.

There is a difference in the Relative Vol Adjust frame between id3v2.3 (XRVA or RVAD) and id3v2.4 (RVA2), I think the later allows for more control of how and where the volume is adjusted I think, but I don't know if the empeg recognizes or uses that frame data at all, or which version.

I'm also trying to find out which version of that frame Mp3Gain uses as I haven't seen that specifically spelled out anywhere from their stuff.