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Abstract There is a strong need to model potential rupture behaviors for the next Cascadia megathrust earthquake. However, there exists significant uncertainty regarding the extent of downdip rupture and rupture speed. To address this problem, we study how the transition region (i.e., the gap), which separates the locked from slow‐slip regions, influences coseismic rupture propagation using 2‐D dynamic rupture simulations governed by a slip‐weakening friction law. We show that rupture propagation through the gap is strongly controlled by the amount of accumulated tectonic initial shear stress and gap friction level. A large amplitude negative dynamic stress drop is needed to arrest downdip rupture. We also observe downdip supershear rupture when the gradient in effective normal stress from the locked to slow‐slip regions is dramatic. Our results justify kinematic rupture models that extend below the gap and suggests the possibility of high‐frequency energy radiation during the next Cascadia megathrust earthquake.