After you apply list to form field then when validation message will be shown out if users input match your item.
For email list only users input match an email in the list you applied can submit for, otherwise will be blocked.
With this example you may do a Gravity Forms registration form only for invited users. Let's suppose you have a campaign and sent invitation to your clients, then you just need import your clients email CSV file into an email list and apply the list to email field in your form.
Email CSV File:
Apply to field:
When submit form:
Gravity Forms will show validation message for the field.