I know the original question is a bit old, but I thought I'd add this:
You can add the following as a custom column (in Playlist view etc). It'll display the ReplayGain track volume in human-speak rather than as a sound pressure level. ReplayGain's 89dBequals -18dB in human-speak. I don't know why we persist using 89dB as a target volume. The rest of the world outside ReplayGain uses a volume that makes sense to humans.
The following all highlight the playing track.
Volume Column (based on track gain)
$if(%replaygain_track_gain%,$puts(X,$sub(-1800,$replace(%replaygain_track_gain%,.,)))$puts(VOLUME,$ifequal($get(X),0,0,$ifgreater($get(X),0,$replace(+$substr($num($get(X),4),1,2),+0,+),$replace($substr($num($get(X),5),1,3),-0,-))).$substr($num($get(X),5),4,5) dB)$if(%isplaying%,>>>$get(VOLUME)<<<,$get(VOLUME)))
Track Gain Column
[$if(%isplaying%,>>>%replaygain_track_gain%<<<,%replaygain_track_gain%)]
Peak Column (based on track peak)
[$if(%isplaying%,>>>%replaygain_track_peak_db%<<<,%replaygain_track_peak_db%)]