Editing / Updating Field value AFTER Submission

Are you wanting to modify the field value after a form has been submitted and the form processing is completely finished? If so there aren't going to be any hooks for that. Once the form processing is complete, there is nothing to hook into.

If you want to modify the field value AFTER the entry has been saved but before form processing is complete you can use one of two hooks depending on what version of Gravity Forms you are running.

Gravity Forms v1.6 (beta) and up:
http://www.gravityhelp.com/documentation/page/Gform_after_submission

Gravity Forms v1.5.2.8 and below:
http://www.gravityhelp.com/documentation/page/Gform_post_submission

These hooks fire after the entry has been created and notifications, etc. have been sent. They are basically the last hooks to be triggered as part of the form process.

You would add a hidden field to your form to contain the tweeted value. You would then use one of the hooks above to update that value of that field based on code you execute. The field has to exist in some capacity on the form in order for it to exist as a field you can update.

Technically you could use the new entry meta, but we don't have any documentation for it yet and it's primarily for internal development right now. It isn't automatically exposed in the admin. You would have to use admin hooks to display it's value.

If you are wanting to update a field value AFTER the form process is completely over then that is an entirely different thing. You'd have to update the form data directly in the database.

It's a different hook and it is the one we recommend using in 1.6+ because we are deprecating gform_post_submission. It won't be removed right now, but may be in the future once a significant amount of time has passed.