Social media marketing plays a relevant role in the brand promotion of enterprises owing to its advantages of rapid and diversified communication with consumers. The Chinese Internet enterprise Alipay launched Ant Forest as a mobile application with gamified social functions, bringing consumer-brand engagement. Ant Forest provides a variety of gamification functions (e.g. point, leaderboard, badge, task and teamwork) to encourage users to participate in environmental protection and public welfare activities. These gamification mechanisms, combined with the spread of social media, have realised the co-creation of user brand value. In the current study, 305 Gen Z users of Ant Forest were surveyed through an online questionnaire, and the data analysis was conducted using the partial least squares structural equation modelling (PLS-SEM) method. This study indicates that the four elements of entertainment, trendiness, customisation and word-of-mouth in social media marketing have statistically significant direct effects on the consumer-brand engagement in Ant Forest. In addition, gamification shows a significant positive moderating effect on the relationship between customisation and consumer-brand engagement and a significant negative moderating effect on the relationship between trendiness and consumer-brand engagement in Ant Forest. This study adds gamification to the conceptual system of social media marketing and provides suggestions for the development of gamified social media marketing applications.