In a rapidly changing environment, it is essential for companies to develop their capabilities for sustainable growth. To this end, informal learning at work plays an important role for small and medium-sized enterprises that are having difficulties in human resource development. If the GRIT of workers and the spiritual leadership of leaders are maintained at an appropriate level, the effect of informal learning will increase. This study derived the following results through a survey of workers participating in the Work-learning Dual System in Korea. First, experience and feedback among informal learning activities have a significant positive effect on competency development. Second, the higher the level of the GRIT of workers, the greater the magnitude of the positive effect of experience and feedback on competency development, showing a positive moderating effect. Third, in the case of workers with high GRIT, capacity development is strengthened as experience or feedback increases, and the higher the level of spiritual leadership, the greater the scope of reinforcement. On the other hand, it can be seen that in the case of workers with low GRIT, competency development weakens as experience or feedback increases, and the higher the level of spiritual leadership, the greater the extent of the weakening.