Alan,
Replaygain is a scheme that objectively measures the loudness of music tracks or albums encoded to flac, ogg, mp3... and adjusts the volume to a standard loudness. This results in reducing the volume for most recordings to minimize the number that would end up clipped, so if an HDCD recording can make use of some of the (digital) headroom, then great. It is handy for mixed playlists or even one album following another, and if your DAC has more resolution than the recordings, I don't think anything is lost.
