I found this thread in google, I was also wondering how to use mp3gain with WMA or OGG files, there is a way, use foobar 2000, its an MP3 player with a built in gain feature
its actually better than MP3gain even for MP3 files since it doesnt mess up the file information tags and it can do WMA and OGG files
in foobar 2000 right click a file or selection of files > Replaygain >Scan per file track gain or scan selection as album
download foobar here
http://static.morbo.org/foobar2000_special.exe
foobar 2000 website
http://www.foobar2000.org/