After you apply list to form field then when validation message will be shown out if users input match your item.
For white list if users input doesn't match a item in the list you applied then it will be blocked.
View Gravity Forms Blacklist and Custom Validation demo
With this example you may do a Gravity Forms registration only for some specific area.
Item:
Comparison:
When submit form:
Gravity Forms will show validation message for the field.
