It's difficult to add a new image to a Gallery Block.
The easiest thing to click is one of the existing images:

Once you have the image selected, you have to somehow know to click this icon all of the way to the left:

Only once you do that does the "Add" toolbar option become visible:

(Ignore the focused caption input, I think that's a bug)
As a user, I'd expect I could easily "Add image" whenever a Gallery Block or its children are selected.
It's difficult to add a new image to a Gallery Block.
The easiest thing to click is one of the existing images:
Once you have the image selected, you have to somehow know to click this icon all of the way to the left:
Only once you do that does the "Add" toolbar option become visible:
(Ignore the focused caption input, I think that's a bug)
As a user, I'd expect I could easily "Add image" whenever a Gallery Block or its children are selected.