That isn’t a caption, its just a text block below the image.
As that image is a GB Image, select it and in its Toolbar there is a add Caption button. Which a) create the figcaption and b) give you the GB styling controls to resize and position it
If you added the caption following David’s advice, you should be able to select the caption block and edit it via the options that pop-up on selection and using the block settings (Typography) in the right-hand pane.