Is there a JavaScript parser to obtain GeoJSON from GML?
I have found only Python and OGR solutions.
I'd use the OpenLayers Format readers and writers.
GML reader here http://dev.openlayers.org/apidocs/files/OpenLayers/Format/GML-js.html
GeoJSON writer here http://dev.openlayers.org/apidocs/files/OpenLayers/Format/GeoJSON-js.html
May be overkill to just convert from GML to GeoJSON, but OpenLayers API are widely used and tested, and also easy to extend.
Also, you could create a small OpenLayers build with just those classes and their dependencies.
I think this project may helps you. https://github.com/highsource/ogc-schemas.
It is based on Node.js environment. but if you want to make use of it in client side, it is possible with Browserify(http://browserify.org/)