How to Generate and Validate a Reference Number for Gravity Forms with GP Unique ID
Generate a reference number on Form A and require this reference number on Form B in order to submit the form.
Require an Existing Value for Submission with Gravity Forms
Ensure that a value entered in Form B has been previously submitted (or generated) on Form A. This is useful if you’re generating a reference number of some sort on Form A and would like the user to enter it on Form B.