Considering how most people will be using the product involves heat (i.e., smoking), GC is the only method that provides accurate, reliable data as to THC concentration. Yes, the THCA is decarboxylated to THC during GC analysis- which is precisely why it is consumed by smoking.
This is also why tinctures created without heat may have little or no medical benefit. Gas chromatography offers the greatest accuracy and precision with respect to quantification of primary cannabinoids, as well as the most reliable cannabinoid profile for those who consume cannabis and its products by smoking. The combination of gas chromatography with flame ionization detector (GC/FID) offers the best combination for quantifying THC, CBD, and CBN, and GC/FID is the method that we offer in our lab.
LC offers the benefit of not exposing the sample to undue heat, which preserves the 'natural' profile of cannabinoids. This allows determination of minor compounds like CBG, CBC, and THCV (among many of the tens of natural phytocannabinoids). However, to the best of my knowledge, nobody is buying or selling marijuana based on its levels of these minor compounds. Moreover, if one were to report the concentration of THC using LC, it would be a fraction of the "real" value, as there is no decarboxylation of THCA to THC. These values must be calculated (versus read directly) if liquid chromatography is used.
Both methods have their advantages and disadvantages; saying one is "good" and other is "bad" gives the science of these analyses a disservice, as does asserting that heat is intrinsically bad for cannabinoids. If it were, nobody would smoke the stuff, or use a vaporizer.