The “real” ReplayGain is built into some media players (for example “Foobar2000” https://www.foobar2000.org/)
Media players from Apple (such as iTunes and iPads) have a similar feature called “Sound Check”.
Some years ago I wrote a kind of “normalizing” plug-in that uses the ReplayGain algorithm so that it can normalize to a specified “loudness”. That plug-in is available here: https://forum.audacityteam.org/t/replaygain/22589/1
This is a “Nyquist plug-in”. Installation instructions for Windows are here: https://manual.audacityteam.org/man/installing_effect_generator_and_analyzer_plug_ins_on_windows.html#nyquist_install