Thin metallic bipolar plates (BPPs) fabricated by stamping technology are regarded as promising alternatives to traditional graphite BPPs in proton exchange membrane (PEM) fuel cell. However, during the stamping process, dimensional error in terms of the variation in channel height is inevitable, which results in performance loss for PEM fuel cell stack. The objective of this study is to investigate the effect of dimensional error on gas diffusion layer (GDL) pressure characteristics in the multicell stacks. At first, parameterized finite element (FE) model of metallic BPP/GDL assembly is established, and the height of channels is considered as varying parameters of linear distribution according to measurements of actual BPPs. Evaluation methods of GDL contact pressure are developed by considering the pressure distribution in the in-plane and through-plane directions. Then, simulation of the assembly process for a series of multicell stacks is performed to explore the relation between dimensional error and contact pressure based on the evaluation methods. Influences of channel number, cell number, and clamping force on the constitutive relation are discussed. At last, experiments are conducted and pressure sensitive films are used to obtain the actual GDL contact pressure. The numerical results show the same trend as experimental results. This study illustrates that contact pressure of each cell layer is in severely uneven distribution for the in-plane direction, and pressure change is unavoidable for the through-plane direction in the multicell stack, especially for the first several cells close to the endplate. The methodology developed is beneficial to the understanding of the dimensional error effect, and it can also be applied to guide the assembling of PEM fuel cell stack.