Having evolved since it's past use of viral and liposome vectors, current gene therapy is done via nanotechnology. Using nano-bots, Abwehrans introduce specific genes into patients to replace old genes of the specific type. This process can be done either somatically or germline (as an adult or fetus).
Germline Gene Therapy is used for a specific purpose only, to prevent genetic illness or severe genetic defects from causing the baby any harm in the future. Though, it was used to try and create 'perfect' babies in the past, laws have been passed to prevent the genetic engineering of children in such manners. Any use of germline gene therapy other than to prevent genetic illness or defects is strictly punished, with criminals often going to prison for decades.
Somatic Gene Therapy is different. It's use is mostly for those who work in extremely hazardous conditions, such as mining on other celestial bodies. It is also rumored, that elite military forces receive somatic gene therapy in order to boost their overall physical performance, but none of these rumors have been confirmed.