After creating invitation codes list and adding / generating codes, you still need to apply list to Gravity Forms form field.
Before apply list to field, you need to make sure you have enabled the feature in forms settings.
Open Advanced tab of the field you'd like to apply list to on Gravity forms form edit page
You may customize the validation message that to be shown in front, if leave it blank then the validation message in form settings will be shown.
Choose action for list
Invitation codes list only have one action of Allow. It means only the field value same as one of the code in the selected list can pass validation.
Update Gravity Forms form to save what you set to the field.Purchase BSK Forms Blacklist Pro