Hello @wpredbranch,
We are aware of two incompatibilities with Gutenberg and the post-to-campaign feature.
The first incompatibility has a workaround. This concerns the SEND button not working. After you click the SEND button, click the UPDATE button above the sidebar and that should create your campaign as before. We’re working on a fix for this.
It’s caused by the structural change from jQuery to ReactJS that powers Gutenberg. The trick here will be to preserve the success/error messaging since Gutenberg doesn’t display admin notices like the old Classic Editor does.
The second incompatibility with Gutenberg (or any page builder, block editor) is that the HTML output it generates contains delimiters and metadata that define each block. When stored some of these blocks add DIV layers that need CSS code to lay them out – usually in a mobile responsive way for browsers. CSS support is more limited in the email clients.
For an example of Gutenberg front end CSS, see the Twenty Ninteen theme file wherever wp-block- is used to style something: https://github.com/WordPress/twentynineteen/blob/master/style.css
My colleague @randywsandberg is working on a compatibility chart to define which blocks work in emails currently, which blocks will require some email compatible CSS for us to bundle, and which blocks we can’t support for emails, such as the embed blocks.
