Gamified Flipped Classrooms (GFC) are increasingly implemented in teaching and have become a trending research subject. Gamification and Flipped Classroom (FC) literature expresses a clear need for a theoretical foundation that positively affects motivation, behavioral change, and learning. However, this requires an overview of the current theoretical foundations of GFC research. Therefore, this study conducted a systematic mixed studies review (SMSR) of the current theoretical foundations of GFC research and learning effects. Sixty-nine studies were identified after applying rigorously defined inclusion and exclusion criteria. The review revealed that a significant number of studies (25 of 69) lacked a theoretical foundation. In studies supported by theory, Self-Determination Theory (SDT) was most frequently employed. Moreover, only three studies used theories to design both gamification and FC components. While GFC research generally shows positive effects on learning, studies with a theoretical background reported a higher rate of positive GFC outcomes (56%), compared to only 29% for studies lacking a theoretical foundation. Future research should explore and attempt to link SDT and other theories to learning impacts.