Action Hooks don’t exist inside the post content. As hooks have to be ‘baked’ into the template code.
However you can create a Shortcode to output a Hook and in turn filter the_content to insert that shortcode.
Very convuluted – and Ying provides the code for that here:
With that code added it will insert a hook after the 2nd paragraph of all posts.
And in a GP Hook element you can select the Custom Hook and add: after_first_paragraph to the custom hook field.