Play is crucial to children's development. With the prevalent use of digital technologies, children and young adults are seen playing games more often and longer on digital devices. How should teachers capitalize the advantages of games in class, without inducing more health risks or problems for children? To achieve this goal, teachers need empirical details and conclusive results in the application of game-based learning (GBL) in primary education, so that they can design GBL experiences that cater to the specific needs of pupils. The present paper provides a systematic review for 35 experimental studies that substantially integrated gaming elements in primary school lessons. Our major findings include that (1) most studies were published in technology-assisted learning journals, focusing on higher-grade students' natural science learning, through one GBL lesson conducted within one week; (2) theoretical foundations were absent in the majority of studies, with the rest implicitly or explicitly drawing upon Constructivist premises; (3) adopted GBL technologies were mostly self-designed, and used for formal learning; (4) Gamification was the most frequently used game genre and Intellectual Problem-Solving was the top gaming element to be incorporated and (5) there were more positive outcomes than mixed findings and more cognitive results reported than affective or behavioral dimensions.