We exploit backward stimulated Brillouin scattering in gases to achieve unprecedented nonlinear optical amplification in fibers. The gain coefficient is 10 times larger than any reported nonlinear gain in gas-filled HC-PCF and 6 times larger than the largest nonlinear gain in standard silica single-mode fiber (SMF). Furthermore, our system can work at any wavelength from vacuum ultraviolet to mid-infrared thanks to the nature of stimulated Brillouin scattering. This massive gain enables us to realize a single-pass gas Brillouin laser with 140 mW threshold power over 50 m of fibre. This platform opens new avenues in gas Brillouin lasing, all-optical signal processing and high performance distributed fiber sensing.