Gene doping will be undetectable by standard anti-doping tests unless a marker is built in.
Here is a Technology Review writeup on a gene therapy that modifies or erases pain with a genetically modified virus injected into your cerebral spinal fluid. I suspect these types of therapies could be on a cheater's radar so I think they should be on WADA's radar too.
Snippet: "...Researchers at Mount Sinai School of Medicine injected a virus carrying the gene for an endogenous opioid--a chemical naturally produced by the body that has the same effect as opiate painkillers such as morphine--directly into the spinal fluid of rats. The injections were targeted to regions of the spinal cord called the dorsal root ganglia, which act as a "pain gate" by intercepting pain signals from the body on their way to the brain. "You can stop pain transmission at the spinal level so that pain impulses never reach the brain," says project leader Andreas Beutler, an assistant professor of hematology and medical oncology at Mount Sinai...." [Link]