Symfonium support replay gain but have a few configuration and different behaviors.
The replay gain values are extracted from the media tags. Supported tags are (case does not matter):
Once you have your files properly tagged you need to configure Symfonium to tell what replay gain value you want applied.
This done in Settings / Playback / Equalize - DSP
Most options are clear, the Automatic mode switch automatically between track and album replay gain value depending on what media you start playing. (Track if you start a playlist or album if you play an album for example).
By default replay gain is only applied when playing locally.
You can enable Replay Gain support when casting to Chromecast with the option Settings / Playback / Enhanced Replay Gain Processor.
This enable the usage of the transcoder to apply the replay gain to the source file allowing RG to work when casting.
Note: This option will be renamed and changed in the future, currently it also affect local playback.