Originally Posted by
mville
There are 2 Replaygain DSPs, ReplayGain and ReplayGain (Apply).
ReplayGain is a non-destructive DSP which writes tags to the files only and DOES NOT alter the audio. These tags are read by media players, which adjusts the audio levels on playback.
ReplayGain (Apply) is a destructive DSP, which alters the audio in a similar way to MP3Gain.
Ideally though, you would use the non-destructive DSP, ReplayGain, but you would have to check that your media player supports reading ReplayGain tags.