Barb, I'm surprised! You are very knowledgeable!
Personally, I feel that to be successful at any eating plan, you HAVE to read the book, and especially so with SBD. There are a lot of basic tenets you need to absorb to try to make good choices, and understanding the why behind many of the rules can really help you work the program. The problem, IMHO, with just using the GFGCG, is that it does not indicate what is "limited" for Phase 2 and good on Phase 3. That can be a tough one to figure out without the book. There is a brief description of the book in the beginning of the GFGCG, but I think reading the book is much more beneficial.
That said, you don't need to buy the book to read it.
Borrow it from a friend or from the library. However, it does have recipes that might be worth having on hand often, so you might want to purchase your own copy if you like them.