For the kinds of gene therapy we currently have, it wouldn't make that much of a difference for society, but could have a positive effect for people affected by conditions that can be treated with it. Although as I understand it, those treatments are far from perfect and do frequently have adverse effects as well.

If we're talking the sci-fi kind of gene therapy that could stop aging, make people more intelligent etc, that would be an entirely different story. It would have massive societal ramifications and I think especially under capitalism, they wouldn't be good. It would increase inequality by massive amounts and basically turn the rich elites into their own, genetically-superior species.