There are very few options to acheive GPEN via self study,

You can look at the course objectives, study them on your own and challenge the exam. This is a tough option but can be done.

http://www.giac.org/reginfo/challenge.php

You need to register with SANS for self study and they will ship you the materials and then register for the exam.

http://www.sans.org/selfstudy/

Or the best option would be to do SANS OnDemand or SANS @Home or VLive.

http://www.sans.org/vlive/

The exam is open book and therefore its best to have the latest SANS material.

Hope this helps.

VJ