A gallery control in Power Apps allows you to display a list of records, and can be configured to sort the records by a specific field, such as category. Additionally, the gallery control has built-in functionality for expanding or hiding a list of subtopics. This can be done by adding a toggle control within the gallery template to show or hide the subtopics based on user interaction. Reference: https://docs.microsoft.com/en-us
/powerapps/maker/canvas-apps/controls/control-gallery