The　stable　vehicular　space-air-ground　integrated　network　(SAGIN)　requires　high-gain　multi-beam/beam-steering　antenna　systems.　In　this　paper,　we　propose　a　compact　high-gain　wideband　circularly-polarized　(CP)　beam-steering　antenna　with　novel　rotatable　phase-gradient　metasurface　(PGM)　transmitarrays　for　the　SAGIN.　It　consists　of　a　novel　substrate-integrated-waveguide　(SIW)　array　feed　and　three　innovative　metasurface　transmitarrays　(M0,　M1,　and　M2),　capable　of　providing　any　compensation　phases　between　0°　and　360°.　The　novel　PGM　unit　cells　have　two　identical　CP　U-slot　patches　that　perform　CP-to-CP　conversions　with　high　transmission　efficiencies.　In　addition,　a　new　and　accurate　beam-steering　analytical　formulations　are　proposed　for　fast　predictions　of　scanning　angles.　Experimental　results　show　that　the　proposed　compact　SIW　array　feed　achieves　an　aperture　efficiency　of　58.8%,　a　3-dB　gain　bandwidth　of　10.8%,　and　a　3-dB　axial　ratio　(AR)　bandwidth　of　over　23%.　The　proposed　entire　beam-steering　antenna　achieves　a　peak　gain　of　27.56　dBic,　which　corresponds　to　an　aperture　efficiency　of　48.5%,　and　an　exceptionally　wide　3-dB　AR　bandwidth　within　the　scanning　range.　It　is　a　strong　candidate　for　future　millimeter-wave　communications　in　satellite　and　radar　systems　and　vehicular　communications.　IEEE