Flac, replay gain and ...... Options

[UncleMcFlac]

Hey , can anyone help me? i use foobar ,and have tons of lossless files, i see the function replay gain but im just learning to use it , heres what ive done to tracks that sound way too low , ive increase the decibels and converted the flac file to flac (click on file > convert to> replay gain > move the preamp together> convert), so when i do that is the process irreversible?
and can someone teach me how to apply replay gain? and explain this thing to me ?
Note: since im a noob , what i do is keep my replay gains between -6db to -9db , there are some track like "one direction - whats makes you beautiful" that are about -12 db(that too darn loud), and the "killer - mr brightside" is that loud too but its not bad because it is rock (rock known for distortion) but the OD song just sound like a hiss fest(the person who produce it should go back to mastering or producing school), also the track i mentioned ive turned them down using the method above but i wanna know if that "technically" degrades the song