A novel two-port antenna employing Gap Waveguide technology introduces +/- 45 degrees slant polarizations for Joint Communication and Sensing (JCAS) systems or polarimetric radars in the 76 - 81 GHz frequency band. The subarray antenna comprises a two-layer radiating structure: the lower part features vertical slots radiating horizontal polarization, while the upper part integrates a septum polarizer and side metallic ridges. The septum polarizer converts horizontal to circular polarization by creating orthogonal electric field components with a 90 degrees phase difference, while side ridges compensate for this phase difference, generating slant-polarized waves. This method achieves slant polarization by exciting the antenna ports without needing cavities to alter electric field orientation. The antenna, with two elements, attains a peak gain of 8.97 dBi within the 76 - 81 GHz frequency band, demonstrating an axial ratio exceeding 30 dB at +/- 45 degrees slant polarization.