This paper introduces a millimeter-wave Integrated Substrate Gap Waveguide (ISGW) filtering antenna featuring four adjustable radiation nulls, with two located in each of its upper and lower stop bands. These nulls are tunable by altering the dimensions of stepped-impedance resonators, complementary U-slots, and passive coplanar parasitic patches. The antenna offers the benefits of individually adjustable radiation nulls and the flexible modification of selectivity and gain curve roll-off. Simulation results indicate the antenna operates at a central frequency of 25.4 GHz, with a relative bandwidth of 14.3% (23.76–27.04 GHz), and attains an average gain of 7.6 dBi.