@marty230498
Strictly speaking, Strawberry does not have the ability to apply ReplayGain. It can READ ReplayGain that has been applied by other software, but it does not include a built in scanner.
That said, there is a new feature in Strawberry, just introduced, called EBU R 128 normalization. This normalizes the volume of tracks, but only within Strawberry. It doesn't create tags in the files' metadata, so it might not work if you need to play the files in another music player.
If you only use Strawberry, you're fine. Just check "Perform song EBU R 128 analysis" in the Collection settings, and then rescan your library. Then enable "EBU R 128 Loudness Normalization" in the Backend settings. Not sure, but I think it might only work on songs actually in your collection; not songs elsewhere on your computer.
I haven't tested this option yet. All my songs are already using ReplayGain (applied by MusicBee when my collection was on Windows).
Now if you want to actually apply ReplayGain to the songs, you will need to use other software. On Windows, free music apps like MusicBee and Foobar have the ability.
I am currently searching for a Linux based software to do this, but I haven't settled on anything yet. Currently testing DeaDBeeF (previously played around with soundKonverter) but I need more information before I decide.
In my opinion, the software one uses to apply ReplayGain should implement it by first scanning, and then writing the analysis data to tags. There are some programs (e.g., mp3gain) that actually change the song files. It may be reversible, but I don't like the idea of changing the actual file. ReplayGain tags just travel along with the files, and do nothing until they are called upon by the player to provide the info needed. The original file remains untouched.
There is far too much information about this topic to get into in this discussion, and, to be honest, I am still learning myself. But I hope this points you in the right direction.