There are also people who don't have the genes but still get celiac. The genetic test actually slowed down our testing/diagnosis process. The first MD we saw was skeptical that my son (then age 6) had it since he had no classic symptoms and was average height and weight. He could not explain the positive blood tests, so he did a genetic test. The genetic test indicated that he was not likely to develop celiac disease, so he concluded that he couldn't possibly have it. (even though I tested positive too, and had ALL the classic symptoms.) This doctor concluded that the initial positive blood test must have been a "lab error".
We continued looking and took him to the Celiac Center at Children's Hospital Boston. They did an endoscopy and found extensive celiac damage.
So in this case, the genetic test results were very misleading. His doctor at CHB doesn't even do them on anyone since they don't really tell you anything. There are exceptions in both directions (some with genes DON'T get it. Some without genes DO get it.) I certainly wouldn't spend any money on it.