Hi
Don't know if I understand the question but
If you select the whole media library mp3 files, right click and choose "send to > calculate Replay Gain", in the end, if you save to files Replay Gain values will written in Metadata for each files for the whole library.
Then if in Preferences > Playback you activate Replay Gain, whatever playlist created on the fly will respect those values written in Metadata.
Calculate Replay Gain for a huge Media library can take a lot of time but it's worth the time
Hope this helps