I haven’t tried the Gravity Forms plugin but if you are using the wp_insert_post function to insert the new post, you just use ‘post_excerpt’.
example:
$new_post = array(
'post_title' => 'Post Title',
'post_content' => 'Post Content',
'post_status' => 'publish',
'post_excerpt' => $custom_excerpt_field
);
wp_insert_post($new_post);
Not sure if I completely understand your question, so I’ve misunderstood something please let me know..