Leaky-wave antennas (LWAs) feature the advantages of high directivity, wide impedance bandwidth, and their ability to scan with frequency. However, the dispersive beam-scanning makes the LWAs very difficult to be utilized in systems that require high-speed data transfer, wide band and high directive antennas. This paper proposes a simple but effective method to reduce the dispersion of LWAs by loading with a metasurface prism realized in gap-waveguide technology. Naturally combining the two technologies, a leaky-wave gap waveguide antenna and a dispersive prism, a low-dispersive gap-waveguide leaky-wave antenna is achieved, with a specific radiation direction that in this example is ϕ= 41°. The main radiation direction varies only ±0.5° from 11.5 to 13.0 GHz, with side-lobe levels lower than −13 dB. A 20% half-power gain-drop frequency bandwidth has been obtained. The realized gain varies from 16.4 to 16.8 dBi with a narrow half-power beamwidth of 5°. The full-wave simulations in HFSS agree well with the theoretical calculations.