would be able to troll your ~/Music folder for files named '*.mp3' or '*.MP3' (case insensitive is -iname) and run mp3gain on all those.
This approach is vastly superior to the -exec option in most cases because it only fires off one mp3gain process, which makes it much faster.
Maybe someone can refute my next claim; I found that adding -type f was almost a necessity, and that the operation was actually faster. Not a lot faster, but fast enough to justify typing it in. Anyone care to weigh in on that one?