This letter proposes a new design approach for filtering antennas. The novel matching reflection coefficient-based method allows the integration of filters and antennas without compromising the frequency behavior of either of these components. Moreover, this integration is done by avoiding the need for lengthy optimization processes and provides a high degree of flexibility in the types of antennas that can be used. In order to validate it, two examples are provided. In both cases, a 4th-order Chebyshev bandpass filter at 101.5 GHz implemented in a stacked groove gap waveguide configuration is used, firstly along with a single aperture antenna and, subsequently, with a slotted ridge gap waveguide array. This second example has been manufactured to demonstrate the usefulness of the new design methodology. Excellent measured performance has been obtained for a filtering antenna at W-band for the first time.