How do I pass a post ID to the page URL?

First of all. The question is a little misleading. What you actually want is “post id(s) for current user”.

Here we go:

// Global variable for current user ID
// More information: http://codex.wordpress.org/Function_Reference/get_currentuserinfo
$user_ID;

// You need to create a new WP query object
// More info: http://codex.wordpress.org/Class_Reference/WP_Query
$my_query = new WP_Query( array(
    'post_type' => 'farmers',
    'author' => $user_ID
));

// You get all the current user posts from the query object
$posts = $my_query->posts;

// You get the first post from the posts array
$first_post = $posts[0];

// You get the post ID from post object and store it to $gform_post_id variable
$gform_post_id = $first_post->ID;

Now you can echo that $gform_post_id in your URL

echo 'http://example.com/dashboard/form_page?gform_post_id='.$gform_post_id;

Cheers