The killer application for gene therapy is enhancement, not disease treatment. Genetic diseases are present in only a tiny number of people, and so the markets that will form there will be miniscule in comparison to those that improve healthy people.

This will play out over the next decade or two.

Look at, e.g. gene therapy for inherited disorders (lysosomal storage diseases, DNA repair deficiencies, etc, etc) versus myostatin / follistatin gene therapy that increases muscle mass, reduces fat tissue, and generally adjusts metabolism into a better direction.

The first companies to commercialize the former will be successful mid-sized biotech concerns. The first companies to commercialize the latter will be behemoths.

The state of development in this field is dependent on methods of delivering genes that are reliable, tissue specific, and achieve high degrees of cell coverage. Once that happens, which seems fairly close at hand, then we'll see a very interesting expansion of the medical tourism marketplace into enhancement technologies, a market hundreds of times as large as therapies for sick people.