As I mentioned to the original poster, you cannot edit the individual Gallery Cards items from within the Hub site. Gallery Cards are dynamically populated. If you wish to change the individual descriptions of the items being displayed in the Gallery Cards, you will need to modify the summary for the item. You can do this from several places.
If your item is in your Hub Content Library, you can find the item, open the Content Details and under Basic Info, update Summary.
If your item is in ArcGIS Online, you can find the item, open the Item Details Page and edit the text to the right of the thumbnail.
If your item is a hub page, you can navigate to the pages tree by selecting the pages icon in the site customize panel, select the page you wish to update, select Page Info, and then fill out the summary.
Page info panel displaying page info and summary
You may find this blog post helpful: https://www.esri.com/arcgis-blog/products/arcgis-hub/design-planning/make-your-content-shine-with-ar...