Unlike grid view, gallery view does not have a Hide fields button; instead, it has a Customize cards button. The Customize cards button lets you determine what fields are visible on your gallery view's cards.
When the Customize cards menu is open, you can see which fields are showing (where the toggle is green and switched to the right) and which fields are hidden (where the toggle is grayed out and switched to the left).
To hide or show a field on the kanban cards, click the toggle next to the name of the field you want to hide or show.
The Hide all and Show all buttons will let you quickly hide/show all fields (except the primary field, which cannot be hidden).
By clicking and dragging the drag handles on the field names, you can quickly rearrange the order of the fields on the cards.
If you have a large number of fields, you can type a search term in the Find a field search box to more easily find the field you're looking for.
You can also hide or show individual fields from the cards in your gallery view by clicking on a card to open up an expanded record.