The gamification design of e-commerce integrates game and shopping tasks to promote consumers’ purchase behaviors. However, it often works inefficiently given the low conversion rate from game participants to product buyers. To better understand the relationship between gamification and purchase behaviors, this study identified two types of engagement, namely game part engagement and task part engagement, and highlighted the mediating role of task part engagement from a nudge perspective. An online survey with 234 consumers was carried out to validate the proposed hypotheses. The results suggested that the relationship between game part engagement and purchase intention was fully mediated by task part engagement, and this relationship was strengthened by task voluntariness. Further, game part engagement was found to be determined by two gamification affordances, e.g., achievement affordance and immersion affordance. This study advances theoretical understanding of the effectiveness of gamification design in e-commerce context by distinguishing task part engagement from game part engagement and theorizing the potential mediating and moderating mechanisms.