Hi Ulrich,
I haven’t had much success with this plugin and Gutenberg I’m afraid. With the plugin loading the editor in a metabox it will display the current excerpt, but I can’t get it to update the excerpt from the contents of the metabox. I believe this is probably down to the fact that Gutenberg allows editing excerpts and will overwrite the excerpt crafted in this plugin’s editor. The mechanism of this plugin is very simplistic, and does not alter the save/update mechanism of the page editor in any way (all it does is replace the textarea for excerpts with TinyMCE).
With the next release I will probably disable this plugin if Gutenberg is enabled until I can find a reliable way of removing excerpt editing from Gutenberg and getting the metabox to save the excerpt instead.
