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A high gain antenna system based on a phase gradient metasurface (PGM) with a four-split ring (FSR) structure is proposed in this letter, which combines the radiation functionalities of transmission-mode, bidirectional-mode, and reflection-mode. The Vivaldi antenna, as a feed source, is placed at the focal point of the metasurface (MS). Then the quasi-spherical wave launched by the feed source undergoes abnormal refraction and reflection when passing through PGM, which proves that the PGM proposed can control the wavefront by altering the frequencies so that the proposed antenna can achieve multimode radiation. In contrast with traditional unidirectional antenna, the proposed antenna has beam converge characteristic in transmission direction at 15.7 GHz with a peak gain of 20.4 dBi, bidirectional radiation at 16.5 GHz, and reflection direction at 17.1 GHz with a peak gain of 16.5 dBi. The results of the simulation and experiment are in good consistency. The proposed PGM has potential applications in designing high-performance multifunctional antennas in communication systems. [ABSTRACT FROM AUTHOR]