Display Other Function Results with Custom Merge Tag

I want to create a custom merge tag that will display the query results from another plugin.

I am looking for a little more insight than what the Docs can provide here (http://www.gravityhelp.com/documentation/page/Gform_custom_merge_tags)

Basically, the other plugin lists favorite posts of the user with the wpfp_list_most_favorited function. How can I create a merge tag that will call this function and insert the results into a Gravity Forms notification if the form is embedded on a page with the plugin's results?

Thanks for the quick response Alex. I am super close, however, I cannot get the function to output the HTML into the notification email. I basically replaced the gform_get_meta with a function wpfp_list_favorite_posts.

I then inserted {download_link} into the notification section but it's not showing up in the notificaiton email. Is there anything else I need to add?

add_filter('gform_replace_merge_tags', 'replace_download_link', 10, 7);
function replace_download_link($text, $form, $entry, $url_encode, $esc_html, $nl2br, $format) {

    //NOTE: this will be the merge tag you enter in the notification text
    $custom_merge_tag = '{download_link}';

    if(strpos($text, $custom_merge_tag) === false)
        return $text;

    //NOTE: replace gform_get_meta() with the function that will return the HTML to be displayed.
    $download_link = wpfp_list_favorite_posts();

    $text = str_replace($custom_merge_tag, $download_link, $text);

    return $text;
}