The phase gradient metasurface has strong abilities to manipulate electromagnetic waves on a subwavelength scale and has a potential to enhance the antenna gain. Based on the single multi-resonance metallic patch srtucture, we propose a new kind of ultra-thin broadband unit cell to manipulate electromagnetic waves and enhance the gain. It has been demonstrated that anomalous reflection can be achieved by utilizing the magnetic resonance between metallic patch and ground plane. Moreover, it is believed that resonance with low quality factor (Q factor) is useful in extending the working bandwidth. In order to extend the bandwidth of phase modulation, it is necessary to design a kind of low-Q unit cell. Besides, we need to extend the phase shift to cover the entire range [0, 360] to achieve the focusing effect. Thus we design a suitable symmetrical unit cell composed of ring and cross metallic patterns to control the phase of reflected waves. The symmetrical structure is useful for decreasing the Q factor so as to get a kind of low-Q unit cell. Theoretically, ring and cross metallic patch can be regarded as multi-resonance unit cells, which can cover the entire scope [0, 360]. The unit cell operates at 15-18 GHz with a thickness of 1 mm and the sides of 0.3 0( 0=20 mm). Furthermore, we design a phase gradient metasurface composed of the designed unit cell to verify the broadband anomalous reflection and focusing effects in CST Microwave Studio; the effect can be clearly illustrated in the simulation results obtained at 15-18 GHz. Due to the successful conversion from plane wave to quasi-spherical wave, we can place the Vivaldi antenna at the focal point of the metasurface as a feed source to transform the quasi-spherical wave to plane wave to enhance antenna gain. The simulation results are in good agreement with the theoretical analysis. Meanwhile, the designed metasurface and Vivaldi antenna have been fabricated and applied to enhance the gain of Vivaldi antenna. Both simulation and test results show that the peak gain has been averagely enhanced by 11 dB during the -1 dB gain bandwidth of 15-18 GHz and the fractional bandwidth is 18.2%. Moreover, due to the thin thickness, light weight and broad band, the designed unit cell may open up a new route for the applications of phase gradient metasurfaces in the microwave band region, and may also used as an alternative of high-gain antenna.