new L.FileGDB(url[,options]); L.fileGDB(url[,options]);
Options are passed to L.Geojson as is. First argument is the url of a zipped file GeoDatabase.
All feature classes in the geodatabase are added to the same layer, if you need more fine grained control you should probably use filegdb.js directly.
The loading events work with leaflet.spin
If you don't know what a File GeoDatabase is count yourself lucky and move along.