Add Salesforce OID to GF form as hidden field

What exactly are you trying to do? You can't add static markup to the forms like this, they are dynamic. If you explain what you are trying to do in more detail we may be able to provide alternative solutions.

You can't change the form action of a Gravity Form, so you can't change the form action to point to SalesForce. Well, I say you can't... you can using a hook BUT once you change the form action, Gravity Forms no longer has control over the form and can't perform any kind of validation, etc. So it is possible, but not recommended.

The proper way to integrate with a 3rd party service is via the use of hooks and custom PHP. You would use the gform_post_submission hook and write custom PHP so that when the form is submitted your code executes and integrates with the 3rd party service API or posts data to it then.

Here is a forum post that discusses their usage:

http://www.gravityhelp.com/forums/topic/sms-api-gateway-need-help-to-get-gform-to-integrate#post-18087

Here is documentation on the gform_post_submission hook:

http://www.gravityhelp.com/documentation/page/Gform_post_submission

Here is documentation on the Entry object:

http://www.gravityhelp.com/documentation/page/Entry_Object

You would write custom code using these and place them in your themes functions.php file to execute them.

Fields can only be added the form using the form editor. So you'd have to add hidden fields to your form using the form editor and use the default values to populate them or use PHP and hooks to dynamically populate their values. You'd then have to use the gform_post_submission hook to post to SalesForce when the form is submitted.