There are basically two goals of Gert's book: list a lot of observations about how people make moral and normative judgements, and defend why this system of judgement-making is basically justified. I think that he succeeded in the first of his goals, but (of course) I'm not so sure about the second. I will note that, regarding his defence of this system of judgement-making, the concepts he uses generally intertwine and are nicely mutually supporting. But it is hard to really evaluate this system given how brief is its defence (about 70 pages).
My main problem with the book is that it was presented as if it were gospel. Perhaps this approach is necessitated by the length of the book, but I would have much preferred an argumentative style that engaged contradicting views.
Also, I don't find lists of how people make judgements very interesting.
Overall, this may have been worth reading, but it is hard to say. I'll probably have to read Gert's Morality: Its Nature and Justification to have any reasonable idea of how good his normative theory really is.