I know in Foobar2000 I can add ReplayGain tags to my SPC files. It won't work on things like NSF/GBS, though. Wish it were more standard in game music players.
Sorry, I should've indicated which kinds I have in my player (I use Winamp, BTW).
For SPC, I have SNESAmp (or Alpha-II SPC Player). For NSF, I have NotSoFatso. For USF, I have 64th Note. And for GSF, I have Highly Advanced. I believe that's all.
Hopefully, there's a way to do track gain on Winamp as well for game music files.
To my knowledge, SNESAmp does volume changes on the fly. Perhaps it's a setting that needs to be implemented, but listen carefully when the songs starts. As I recall (it's been awhile), it starts out louder, but quickly gets somewhat quieter.
NSF = None that I can think of.
USF = 64th Note gives you the option of changing the volume using the tags. I'm pretty sure there's an algorithm you could use to translate a track gain, to the volume level you'd need to set in the tags. Highly Experimental has ReplayGain implemented, and I think it goes through a similar means.
GSF = See USF. They both have volume tags.
So, loosely, I think it can be done. However, it will probably require some math skills on your part. Sorry I can't help more than that though. Mouser X over and out.
In the playlist editor in Winamp, right-click an SPC file and go to View File Info. In the dialog that comes up (assuming you're using SNESamp), go to the Extended tab. You'll see "Amplification". This is the SPC equivalent of Replaygain.
To get the proper Replaygain value for a file, enable SNESamp's control dialog (in the SNESamp configuration) and play with the leftmost slider in that dialog. The value at the top of the slider is the value to save in the file's tag.