It absolutely is gene therapy! You genetically modified the T cells to make them behave in a new way, and attack a new target.
If you did not introduce a new gene into the T-cells, there would be no therapeutic benefit.
Consequently, it is gene therapy.
The patient can't pass the genetic change to anybody else, let alone their offspring, and the genetically modified cells won't last forever, even though they can replicate, but that doesn't mean it isn't gene therapy.