Improving and controlling the efficiency of a gain medium is one of the most challenging problems of laser research. By measuring the gain length in an opal-based photonic crystal (PhC) doped with laser dye, we demonstrate that optical amplification is more than twenty -fold enhanced along the Γ-K symmetry directions of the face centered-cubic photonic crystal. The samples under study are dye doped thin-film artificial opals, from which gain is extracted by using the stripe length technique: the spontaneous emitted photons, traveling in the direction of the pumped stripe, undergo an exponential amplification of the intensity as they pass through an area of population inversion. This can be quantified by the gain coefficient G which we demonstrate do depend from the propagation direction.