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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.