An　optically　transparent　antenna　(OTA)　based　on　a　grating　metamaterial　is　proposed　for　launching　high-gain　millimeter-wave　radiation　on　glasses.　This　design　incorporates　a　glass-based　dielectric　image　line　with　a　grating　made　of　a　transparent　conductive　film　(TCF),　which　provides　an　effective　near-zero-index　response　through　precise　manipulation　of　spatial　harmonic　dispersion.　To　effectively　overcome　the　intrinsic　loss　issue　of　TCF　and　control　the　power　leakage,　the　field　confinement　in　the　grating　metamaterial　is　engineered　for　optimizing　the　purely　leakage-induced　and　total　modal　attenuations　at　the　zero　phase-progress　state.　The　prototyped　grating　metamaterial　antenna　features　an　average　optical　transmittance　of　76%　over　the　visible　spectrum.　The　antenna　is　validated　to　generate　a　near-broadside　directive　beam,　achieving　a　realized　gain　exceeding　23　dBi　and　a　total　efficiency　higher　than　60%　from　21.1　to　22.3　GHz.　Through　leaky　mode　confinement　control　and　complex　dispersion　relationship　engineering,　this　work　offers　a　promising　solution　to　realizing　OTAs　with　a　substantially　improved　gain　and　aperture　efficiency.