Interesting, I guess I should have done some testing with MP3 files
The problem with MP3 is that to have true gapless, you need to add extra ID3v2 tags to them to point out where the track actually ends (due to a limitation in the MP3 format). MC handles this nicely, of course, but Gizmo/WebGizmo probably wouldn't, because they're dependent on the browser/phone OS... Of course, if you have a single file/stream, that's not a problem, hence my suggestion.
Vorbis is a nice alternative, but most frameworks probably don't support it natively. Perhaps AAC?
In any case, the gaps can be at least significantly reduced by some platform-specific tricks. In Android, they finally added an option to the framework in Jelly Bean (4.1):
http://developer.android.com/about/versions/android-4.1.html#GaplessIn WebGizmo, you could use some of the newer HTML5 audio APIs to separate streaming and playback, so that you can start streaming the next file before the current one finishes, ensuring a smoother transition. Or you can just use an existing library like jPlayer or SoundManager, which should have functionality for that as well, and gracefully switch between Flash and HTML5 where available.