I took and passed the CGRN exam at the SGNA conference in Nashville 3 (?) ago. My study material was almost exclusively the SGNA Core Curriculum which can be purchased from the SGNA or at any large bookstore. If you can't find it at your local Borders or similar store they can order it for you. I'll bet you can also buy it on Amazon.com although I havn't checked. Also, when you apply to take the exam you have the option of purchasing the Core curriculum as well as a study guide and a sample test which was also helpful. It was a difficult test, but not impossible if you've studied and been working in the GI field for a while. I'd been working in the endo lab for about 3 years when I took the exam.
It hasn't affected my pay although the hospital where I work has a career ladder which accepts certification as part of the portfolio if I were to apply for a step up in my job classification.
I took the job mostly for my own professional advancement. I am a preceptor for new hires into the endo lab. I'm also one of a handful of nurses in our lab who performs esophageal manometry and 24 hour pH studies. I'm learning ano-rectal manometry but it's slow learning due to small demand. It's hard to learn it when I do them 6 months apart!
Another reason I took the exam is that I have an ADN degree and don't really want to go back for my BSN. (I have a BS in English...who the heck needs 2 bachelor degrees?
Anyway...I'm not sorry I took the test, but it is a bear trying to keep up with the GI CEU's for recertification in a couple of years.