For use as a voting tool

1) User Registration is supported with the User Registration Add-On that is available with the Developer License. It does support custom profile fields. Just be aware it only creates the user, it doesn't act as a profile editor and doesn't add those fields to the profile. It only creates the data on initial registration.

2) Gravity Forms v1.6 (currently available as a development release) has a new option to lock a form down so that only logged in users can view it. If you want to limit a form so that a user can only submit it one time... that is a little trickier. There isn't a built in option for this. But it makes sense so I will add it to our to do list. Until then it could only be accomplished via a customization.

3) The form would capture any data that is entered. Just be aware that it's a single form. Submitting one form doesn't incorporate answers and fields from another form. They are self contained.

4) Gravity Forms entry data is stored in your WordPress database and can be viewed/edited in your WordPress dashboard. It can also send email notifications containing the data. If you need to export the data it can be exported as a CSV file using the built in import/export tool.

If you want to capture profile information on subsequent forms you can add hidden fields to your form and then use available merge tags to store user related data in those fields using the available merge tags in the default value of each hidden field you add.

1) Yes. If Jeff, Johnson, 47, Iowa is user meta associated with the user submitting the form you could use the user meta merge tag in the default value of those hidden fields to populate those fields with those values.

2) You would have to use PHP to display the form. You'd have to first query the Gravity Forms entry data for entries for that form from that user id. If none exist, you'd use the Gravity Forms function to display the form. If one exists, then you wouldn't and could show a message or something if you wanted to. It's custom code so you can handle it however you want. So it depends on how familiar you are with WordPress, PHP and MySQL.

3) If you are referring to setting up the ability to capture data across all forms on a page... i'm not sure how that is even possible as each form is self contained and when you submit one it only submits that one form. It's just how forms work. Not just Gravity Forms, web forms in general.

We don't do custom work ourselves. But if you need custom work done we'd be glad to refer you to WordPress developers that do.