We study the energy scaling of terahertz (THz) emission through difference frequency generation of near-infrared pulses,and demonstrate that Gigawatt few-cycle THz transients at the central frequency of 30 THz are produced from GaSe crystal pumped by two pulses at 1.65 and 1.95 micrometers,with the high quantum yield of 28％.Our analysis indicates that the high yield of DFG originates from the largely reduced group velocity mismatch as the long-wavelength pumping pulses are employed.