These genetic tests are just diagnostic indicators and the best "suited" psychotropic drugs after the DNA testing are still slightly at less than 50% response, however, much higher than drugs (on the average) prescribed based only on clinical assessment.
I'd say, if insurance covers it, it might be worth trying GeneSight, without putting very high hopes on it.
Keep in mind that there are other factors involved, such as nutritional deficiencies, fitness, breathing pattern, past traumas, diet and other lifestyle/environment influences, which could be addressed with other means outside prescribed medications.
Look into Lifestyle Medicine for more details.
I don't know about this process, but another process used is looking at your liver metabolites -- this indicates whether a drug is likely to be metabolized well or not. Another technique used is to take scans of your brain activity and compare it to other scans. Another used is an EEG test. But my own view is that none of these actually work the way your therapist thinks they do -- after you take the test, you try the drug, and if it doesn't work, you try another -- sound familiar? My recommendation, though, is if you can afford it spend your money instead on what's called a functional physician -- a psychiatrist who actually practices medicine. They will do tests to rule out all possible physiological causes for your problem, and they can also do these tests such as liver metabolite tests if they rule out physiology and determine it's a mental illness. They'll also have a grounding in integrated medicine, which LightSeeker refers to when he refers to lifestyle medicine, which will include looking at your diet, exercise, meditation, and the like. But if they do agree medication is the best course, whenever you get to that point eventually it becomes trial and error because medication doesn't treat the cause of the problem, it only treats the symptoms, because we don't know yet what causes mental illness in most cases, and because individuals just differ in so many different ways that even if there's anything to genetic testing my guess is people probably still have different side effects and metabolism of medication that prevents generalization. If you have the money, though, it never hurts to try something that isn't invasive.