Can't find any instructions to post form data to external url

What exactly are you trying to do?

Gravity Forms posts to itself and stores the data. After it posts to itself you can tell it to redirect elsewhere, but it would do so via query string parameters rather than POST (so more like GET rather than POST).

To do that you would:

- Edit the form
- Edit the Form Settings
- Select the Confirmation tab
- Choose Redirect as the confirmation type
- Enter the address the form should redirect to
- Check the "Pass Field Data..." checkbox
- Build the query string using the form field variable drop down.

To build the query string you would for example type in that box the query string and where you want the form field values output you would select that form field from the "Insert form field" drop down. This inserts a token that will output the field value.

If you absolutely need to post data what you can do is use this redirect and redirect to a PHP page that parses this data via the query string and then posts it.

The other option would be to use post submission API hook and post the data when the form is submitted, but that would require more PHP knowledge and how to use WordPress hooks.

Try the following code snippet. Place it in your theme's function.php file. I added some fields as an example, but you will need to add all others. Good luck!

add_action("gform_post_submission", "post_to_crm", 10, 2);

function post_to_crm($entry, $form){
    if($form["id"] != 3) //NOTE: replace 3 with your form id
        return;
    ?>
        <form method="post" action="http://crm.zoho.com/crm/WebToLeadForm" name="form_to_crm" id="form_to_crm">
            <input type="hidden" name="name" value="<?php echo $entry["1"] ?>" />
            <input type="hidden" name="email" value="<?php echo $entry["2"] ?>" />
            <input type="hidden" name="phone" value="<?php echo $entry["3"] ?>" />
            NOTE: $entry["1"] will return field with ID=1. You can view the field ID by inspecting that input's markup
            .... add your other fields here

        </form>
        <script type="text/javascript">
            document.getElementById("form_to_crm").submit();
        </script>
    <?php
}

To make the checkbox work, you still want to store the value in a hidden field.
<input type="hidden" name="Mailchimp-Newsletter" value="<?php echo $entry["12.1"] ?>" />
What I am not sure is what the CRM expects the value of this field to be. (True/False, Yes/No, 0/1)? Try the following and tweak the "true" and "false" values to see which one works.
<input type="hidden" name="Mailchimp-Newsletter" value="<?php echo empty($entry["12.1"]) ? "false" : "true" ?>" />

As far as multiple forms, you can just duplicate the code and change the appropriate form id and field ids.

From looking at your form you would use 12.1 like in Alex's example because that is the field id for the newsletter checkbox. His PHP code is what you should use, although he mentions you may need to play with the location of the true/false values in his code (ie. try swapping their order if it doesn't work).