Why custom widget won’t appear in Block editor (Gutenberg)?

Only Blocks appear in the block editor. A recent version of WordPress added the ability to add Blocks to widget areas, but not the ability to add Widgets to posts.

You have registered a Widget, which can only be added to widget areas. To create a widget that can be added anywhere that Blocks can be used, you need to create a Block: https://developer.wordpress.org/block-editor/handbook/tutorials/create-block/