Pankaj,
This looks like a great project and I would be very interested in using this in our Liferay. If you'd be interested in collaborating or sharing this project, please let me know.
I think I found one problem. On line 37, you call getDocumentLink() which appears to be an invalid method. If you look at the API documentation here:
http://code.google.com/apis/gdata/javadoc/com/google/gdata/data/docs/DocumentExportEntry.html it looks like the method you want is getDocumentExportLink().
So, the full line should be valid:
Give that a shot and see if it helps. If not, I have a couple of other ideas that seemed less feasible.