appliedReplayGainDb

The ReplayGain actually applied to the current track, in dB, or null when none is.

Null covers three different situations on purpose: the feature is off, the media carries no usable tag and no fallback was configured, or no audio track is open. What it never means is "the tag said zero": a file measured as needing no change publishes 0, not null.

It reports what was APPLIED, so a gain the peak clamp reduced shows the reduced figure and not what the tag asked for.