I found the small drop down arrow in the Media and Text block and in the Pargaraph block, but not in the Classic block. Here are three attempts to do what I want and none of them worked:
http://mofsa.org/test2/board/
By contrast, on another test website with the Classic Editor plugin, text flowing around image works just fine:
http://www.mofsa.org/test/board2/
In the classic editor there are resizing handles around the image, but in Gutenberg there is only a control to numerically set the width of the image.
So my question about how to get text flowing around an image in Gutenberg is unanswered.