ReplayGain is supported by MPD and, hence, by the BDP-X.
You need to access the Samba share called "user" that takes you to /mnt/img.
Add line "replaygain;album" in <root>/mnt/img/mpdconf.set
Reboot the BDP and the setting will be applied to the running MPD.
I've suggested making that a Manic Moose MDP setting, in the past, but this is how you do it on your own.
Of course, your audio files must have the appropriate tags - MPD won't calculate them for you.