Just because this experiment utilized genetic knockouts does not mean a therapy derived from the data would require that.
The gene removed from the mice is the blueprint for manufacture of a protein, in this case a potassium transporter involved in serotonin release and production. The results suggest that drug companies should explore creation of drugs that can inhibit the action of this protein, perhaps by binding to it and blocking its transport of potassium.
The use of gene therapy in depression would be quite controversial, and instead the creation of new drugs that can target this protein is the much more likely result of this finding.