There is a pressing need for gender inclusive approaches to engage young people in computer science. A recent popular approach has been to harness learners' enthusiasm for computer games to motivate them to learn computer science concepts through game authoring. This article describes a study in which 992 learners across 13 schools took part in a game-making project. It provides evidence from 225 pre-test and post-test questionnaires on how learners' attitudes to computing changed during the project, as well as qualitative reflections from the class teachers on how the project affected their learners. Results indicate that girls did not enjoy the experience as much as boys, and that in fact, the project may make pupils less inclined to study computing in the future. This has important implications for future efforts to engage young people in computing. (Contains 2 tables, 5 figures and 8 notes.)