In Gene therepy the desired Gene copy is been replaced with viral dna sequence and introduced into human,results in editing the desired sequence into human dna,but this is risky as some times human immune system will treat them as Intruders.
As far as I have read about gene therapy, isn't it majorly used to treat severe immunodeficiencies? I have read that the pathogenicity of the delivering viruses are greatly reduced prior to be using used. A good question though; However, your question needs expert advise from researchers in this forum.
Typically, the patients are subjected to conditioning regimens pre-treatment. And are maintained in strict immunosuppressants until the therapy is deemed successful.