Game-based learning (GBL) and gamification can improve the learning experience of students by making learning more fun, interesting, and motivating. However, integrating games in practice is challenging for many teachers as it requires competences that not necessarily are part of their teaching repertoire. Game-based pedagogy (GBP) refers to the teaching methods and learning processes involved in learning with games. Research stresses the need for adequate professional development and teacher education on GBP. However, there is a lack of empirical knowledge on effective methods to prepare pre-service and in-service teachers for using game-based learning. The aim of our research is to gain insight into the design of effective GBP learning experiences for teachers. The guiding research question was: What design elements of a course on GBL impacted in-service teachers' GBP competences and teaching practice? We investigated this question in the context of a teacher education program in the Fall 2023. We conducted an empirical study in which a course on GBL was designed, implemented, and evaluated in practice. The participants were 16 in-service secondary teachers from different disciplines in secondary education, from which 13 agreed to participate in this study, and three course leaders. We investigated participants’ and course leaders’ experiences, participants’ competences in GBP, the impact on participants’ teaching practice and the way design elements contributed to it. The data consisted of participant reflections, transcripts from participants and course leaders’ interviews and answers to a questionnaire. The data was collected and analysed using quantitative and qualitative methods between January and April 2024. Results reveal that in-service teachers’ improved their competences on GBP and increased their use of GBL in practice. Qualitative data analysis provides insight into the course's design elements and on participants’ learning process. This study contributes to GBP-education by offering a possible design solution and framework for developing effective teacher education.