Thermochemical gasification offers an attractive solution for the conversion of low-grade biomass and waste. However, practical experiences of the gasification processes reveal that the formation of tar is troublesome to continuous operation. Therefore, tar measurement protocols and tar reduction systems are priorities in the development of effective biomass gasification. Results of tar measurements often raise questions regarding their reliability and accuracy, because of calibration, sampling, and discrimination issues. The present work evaluates the solid phase adsorption (SPA) gas chromatography (GC) measurement system for tar in product gas by comparing the mass spectroscopy detector (MSD) and flame ionization detector (FID) and their associated measurement uncertainty. The measurand is defined as the total GC detectable tar in a normal cubic meter of dry product gas when employing the common quantitation method, "quantitation as naphthalene". The GC-FID measurements were significantly higher than the GC-MSD measurements. Their overall uncertainties also vary by a significant margin. The measurement uncertainty analysis shows that this difference is taken into account by the uncertainty induced by the particulars of the GC-MSD and GC-FID measurement systems, where the relative expanded uncertainty is shown to be 109.4% and 35.0%, respectively. While a quantitative method based on a single calibration curve offers significant advantages, in terms of speed and simple quantitation of total GC detectable tar, such an approach introduces greater uncertainty within the reported results.