The gutenberg_node_type_route hook is not working

The problem indeed exist. Without a return nothing will happen. But why do you have the assumption in your patch that hook_gutenberg_node_type_route() returns an array? Shouldn't that return simply the content type as a string?

No, you're right. Since it's not an alter, but invokeAll. The the approach is actually absolutely alright.

I still have problems to see how this can work. getGutenbergNodeTypeFromRoute

if ($node_type) {
      // Then just return it.
      return $node_type->get('type');
    }

$node_type is group:request for me and since the condition does not check if it's an NodeType object and fails with

Error: Call to a member function get() on string in Drupal\gutenberg\GutenbergContentTypeManager->getGutenbergNodeTypeFromRoute() (line 120 of modules/contrib/gutenberg/src/GutenbergContentTypeManager.php).

#2 works for me, we might just make it clearer in the example hook, to use entity.group_relationship.create_form instead of entity.group_content.create_form and ensure that the plugin is from group_node. Updated the patch with this.

Also, not sure if we should account the weight of the hooks here, but taking the first available hook that implements it for a Gutenberg enabled node type looks good.

For #7, it might indeed be that the route received a scalar parameter instead of the object because the route probably does not implement options parameters. So we could just check for NodeTypeInterface and NodeInterface here.