Yes, it's a virus, specifically a lentivirus, which inserts its own genome into the cells DNA, including the therapeutic gene. Looks like they first extract bone marrow from a patient, apply the virus which adds the gene, then they put the "fixed" bone marrow cells back into the patient (probably after killing most existing bone marrow through some really unpleasant procedures). The re-inserted, treated cells then expand and begin making new cells that have inherited the fixed gene.

In general, gene therapy is probably most useful currently for diseases that affect the bone marrow (and the eyes), because these are two tissues where you can precisely deliver the virus. It's still challenging to control where the virus goes and which cells it infects if you just inject it straight into the bloodstream.