How to float an image in Gutenberg

There are two methods, which can be used:

Method 1

Add Classic block, and insert, as the only element, a right aligned image, into that block. Add other blocks, after the Classic block.

Method 2

Add Image block, and add alignright class, in Advanced setting of this block. Follow that Image block by other blocks.

I also discovered you can’t insert an image from a relative URL in
Gutenberg.

Media Library always provides full URL of the image. After the image is inserted, switch from Visual Editor to Code Editor, and edit src attribute of img element to use a relative URL.