How do I post to a URL

Gravity Forms isn't designed to post to a 3rd party service. It's designed to be integrated with 3rd party services via custom code and API hooks.

It is possible to change the post action and point it to the 3rd party URL, however one thing to keep in mind is if you do this you both Gravity Forms form field validation (because you have changed the form action) and Gravity Forms would also no longer store the entry or send email notifications because it no longer would control the form action.

If you absolutely have to change the form action you can do so using custom code and the gform_form_tag hook. You would add the custom code to your themes functions.php file.

Here is an example of the gform_form_tag changing the action:

<?php
add_filter("gform_form_tag", "form_tag", 10, 2);
function form_tag($form_tag, $form){
    $form_tag = preg_replace("|action='(.*?)'|", "action='custom_handler.php'", $form_tag);
    return $form_tag;
}
?>

The code above was an example, you would have to customize it to suit your needs and those of the 3rd party you want to integrate with.

We don't do customizations ourselves as our hands are full with product development and supporting our product. Here are some WordPress consultants you could contact who have Gravity Forms customization experience that may be able to assist you:

Ounce of Talent
http://www.ounceoftalent.com

WebDev Studios
http://www.webdevstudios.com