Zeolitic imidazolate frameworks (ZIFs), a subclass of metal–organic frameworks (MOFs), have been widely investigated as additive in mixed matrix membranes (MMMs), but systematic studies to identify critical intrinsic ZIF-type parameters that determine the MMM performance are lacking. Therefore, three isoreticular gmelinite (GME) ZIFs, i.e., ZIF-68, 69 and 78 that are distinct by their partial different imidazolate linkers, were incorporated in Matrimid matrices to systematically elucidate the effects of the varying pore aperture, pore diameter, pore volume and functionality of the ZIFs on the CO2/N2 and CO2/CH4 MMM mixed gas permeability. The incorporation of the ZIFs in Matrimid increases the CO2 permeability for all MMMs for both CO2/N2 and CO2/CH4 feed mixtures without compromising the selectivity. The highest increase in CO2 permeability is observed for the 20 wt% ZIF-68 MMM, with an increase in CO2 permeability of 116% for CO2/N2 feed and 122% for CO2/CH4 feed relative to Matrimid was achieved and surpassed the permeability of conventional ZIF-8/Matrimid MMMs. Regarding the isoreticular ZIF series, the results showed that for a higher permeability a larger pore aperture, diameter and volume of the ZIFs in the MMMs are more beneficial than a more polar functionality with smaller pore size, aperture and volume.