ReplayGain in FLAC

I would like to know, how does ReplayGain works on FLAC file format?

I have been using MP3Gain. With it I can pump up the volume and revere that process without quality loss. But I use it for the opposite: if I have to edit and MP3 file, which happens to be pumped up, I can lower it down before encoding it to WAV for those editing purposes. In a matter of fact, I do this every time for new MP3 files [because I always do some editing on new files, so it's more efficient for me to do unnecessary RG operation for around 90% files that to go back for those 10% later on that do really need it]

Unfortunately MP3Gain does not works with FLACs [or other files]. So I have three question:

1] Does ReplayGaining on FLACs works the same way? I mean: if a FLAC file is pumped up, do I have to [should to] lower it down before encoding to WAV [just like MP3]? Or maybe that gain info is stored somehow differently in them; and so audio in WAV won't show up raised as it is in FLAC [from which that WAV was created]?

2] Does ReplayGaining on WavPacks works the same way? Do all file files are treated the same way with this process?

3] What [preferably as simple as MP3Gin] software could I use to check / change replay gain on FLACs [and maybe some other files]? All I found was foobar2000, but it doesn't seem to work neither for FLACs and MP3's ["remove ReplayGain information from files" just do not affect my MP3, which I can change with MP3Dain

And here are the answers: