Awareness of environmental protection with respect to the particulate number (PN) in the exhaust emissions of gasoline direct injection (GDI) engine vehicles has increased. In order to decrease the emission of particulate matter (PM), suppressing emissions by improving engine combustion, and/or filtering PM with a gasoline particulate filter (GPF) is effective. This paper describes the improvement of the coated GPF to reduce pressure drop while securing three-way performance and PN filtration efficiency. It was necessary to load a certain amount of washcoat on the GPF to add the three-way function, but this led to an increase in pressure drop that affected engine power. The pressure drop was influenced by the gas permeation properties of the filter wall. To understand the influence factors of wall permeability, investigations of the pore information of the filter wall were carried out with mercury porosimetry and a three-dimensional structure analysis of the pore structure using micro X-ray CT. As a result, it was confirmed that the number of percolation paths was decreased significantly by the washcoat loading. In addition, the contribution of the diameter of each percolation path was analyzed by applying the permeability model. It was clarified that the number of narrow percolation paths influenced permeability. A washcoat method that controls the washcoat distribution was applied to increase the number of percolation paths, and a coated GPF with low pressure drop was developed.