RGAD - Replay Gain Adjustment (class 3)

Specified at Hydrogen Audio

        <Header for 'Replay Gain Adjustment', ID: "RGAD">
        Peak Amplitude                          $xx $xx $xx $xx
        Radio Replay Gain Adjustment            $xx $xx
        Audiophile Replay Gain Adjustment       $xx $xx

        Header consists of:
        Frame ID                $52 $47 $41 $44 = "RGAD"
        Size                    $00 $00 $00 $08
        Flags                   $40 $00         (%01000000 %00000000)

        In the RGAD frame, the flags state that the frame should be preserved if the ID3v2 
        tag is altered, but discarded if the audio data is altered.

This is not widely supported and I think it has been superseded by RVA2 in ID3v2.4 (and the XRVA tag for 2.3 compatibility).