Hi
I can also reproduce the issue in Power BI Desktop latest version 2.39.4526.362. The GapAnalysis visual will only display 20 group instances, 40 rows in dataset. As I see the visual is updated on 4/14/2016 which is long time ago. The visual needs to be updated to keep up with Power BI Desktop. For this issue, please contact the author of this visual for help. You can go to Visual Gallery, click Contact Author button like below:
Best Regards,
Qiuyun Yu