Game-based learning is quickly becoming a popular trend in Technology-Enhanced Learning. However, the field is very broad with many different initiatives being classified as game-based learning. On the other hand, instructors are demanding effective ways to track the interaction of the students with the games and to assess the learning process. The diversity is a major issue in this regard, requiring instructors to understand each game and to evaluate different kinds of games in different ways. In this work we present a unified mechanism to gather tracking and assessment information from different and varied games. All the information is stored in an online learning environment, where the instructor can consult it. The keystone of the approach is a tracking and assessment API that can be implemented by different games on the client-side and by diverse online learning environments on the server-side. This approach is illustrated with the <e-Adventure> family of educational platforms, which support this interoperable API.