To have the most general idea about the principles of gene therapy you can watch this video
The lecture by Dr. Hans-Peter Kiem from Fred Hutchinson Cancer Research Center at University of Washington provides more detailed information about the main approaches utilized by gene therapy, nucleic acid delivery methods into the cells and also the diseases that use gene therapy for treatment
Dr. John Hoffman’s talk is devoted to sex cells gene therapy to prevent or treat conditions in the offspring
John Hoffman also talks about the history of gene therapy in his other lecture
You can also check out the lectures devoted to gene therapy of particular diseases. Nick Leschly, CEO of the Bluebirdbio company, that develops and commercializes gene therapy treatments, gives a lecture regarding adrenoleukodystrophy
Here’s the talk by Andrew Davidoff, head of the Department of Surgery at St. Jude Children’s Research Hospital, about hemophilia B gene therapy
A brief talk by Nobel Prize laureat David Baltimore about HIV prevention gene therapy
Roger Hajjar, Director of Cardiology Research Center at Mount Sinai Hospital, is lecturing about gene therapy for treating cardiovascular diseases