The GSEC exam is based on the courseware by SANS. All answers come from the books. I took it twice. Once in 2006 and again in 2009 (recert). All based on the books. But - real life experience is also a plus. Since it covers a wide variety of topics (as it is the foundation course for going to 500 level), I would say you will definately need the material unless your knowledge is great in multiple areas.
There may be study guides that are available. Also - you may want to see if someone may have a practice exam you could have. They give two and usually, someone only uses one (I did and gave my other away last year).
Hope that helps a bit. Anything else you'd like to know, please ask!
GSEC - GCIH - GSNA - GPEN