The thing is that mp3gain processes, as it name says, mp3.
Resonic is a multi-format player :
WAV, AIFF, FLAC, APE, ALAC, WV, TTA
MP3, MP2, MP1, OGG, M4A, MP4, AAC, MPC, MP+, OPUS, SPX
WMA, WMA Pro, WMA Lossless
IT, XM, S3M, MTM, MOD
and even embed videos audio.
Mp3gain cannot be the solution. It just cannot compete close to that diversity of formats.
Despite the format issue, i wanted to remember that all kind all normalization are integrated in players, since years. Short time average level (real time analysis), average level over the entire file, peak levels etc...
If there is such a huge amonts of players with it, it because there is a real demand about average levels between files, to not be disturbed (example if you have a playlist on a party). I don't say it has to become the default setting : an on/off button is perfect (in a sound analyses perspective, it's important to have the opportunity to set that off

) It's the stronger attack against the loudness war we can imagine.
I think that
average level in real time (with no cache) based on the entire song is the best solution : it doesn't modify the dynamic of the original file, no loss in quality, and no extra metadata/cache files, and all formats can be supported. That's it

Sure there can be different average levels between files, but no solution can't be perfect. That said, Spotify and Itunes have it (or similar function), other players will also have it. Why not Resonic ?
If Ian Sherperd and Bob Katz hadn't convince you, I don't think I will, but you have my point.
And Tom too
