Actually it doesn't in detail. But there are some good reasons for this.
We do point that out in the book, as it is quite a common question for people coming to GWT.
But rather than GWT looking deficient in this area, it is acutally really flexible. You can write your server side database access code in whathever language / technology you prefer.
Unfortunately there were page count restrictions on the book, otherwise I am sure Rob and I could easily have gone over 1000 pages So, in the book we chose to devote around 4 chapters to focus on the various ways of communicating from client to server GWT provides (HttpRequest, RequestBuilder, RPC) letting the details of database connection be covered by books better suited to those questions.
Would be interesting to know if you think this was a good approach or not?