This is put in the functions.php file of the theme and pulls the content of the post that the form is in to dynamically populate a specific GF field.

get post content to gravity form field

//points to the gform field where you've named the variable post_body
add_filter('gform_field_value_post_body', 'populate_post_body');
//fills the post_body variable with the value found in get_the_content
function populate_post_body($value){
	global $post;
	$post_body = get_the_content();
	return $post_body;
}