Hello all! I like replaygain for pop music, so I am here to share with you the ultimate solution I found.
Which is to utilize the script provided here by this blogger:
Necessary installations for command metaflac:
I decided not to use the plug in because there is too much that I have to mess with, including downloading external executable and pointing picard to use it, saving problems etc.
My recommendation instead of using the individual tools like mp3gain etc. would be to use a single more modern tool like loudgain (which I personally use) or regainer.
Multiple people have also talked about updating the replaygain plugin by integrating the functionality of either loudgain or regainer directly into Picard, but so far nobody has actually done it