To modify the separation between cards, use the spacing argument. Currently this argument is numeric, and takes a number between 0 and 5. At spacing = 0, all spacing between cards is removed:
cards(data = galleries[1:6, ], image = image_url, spacing = 0)
At spacing = 5 there is a lot of room between cards:
cards(data = galleries[1:6, ], image = image_url, spacing = 5)