Our plugin Gravity Forms Blacklist and Custom Validation has the features of blacklist, white list, email list and IP list. It also supports Gravity Forms huge whitelist.
For some case you may have lots of customers and the list maybe huge. DO NOT WORRY about our plugin, as we have tested it with 3000 items on a email list and every email address with 64 chars. We also put a blacklist with 3000 records together with the 3000 email list and it worked well.
Blacklist to let you set keywords to block form submission. White list let you built list list and only allow specific value can be submitted. Email list support to allow or block email which exist in your list. For email list an important uses is building an invention to register form. You may build your customs emails in CSV and then imported into email list and apply to your form's email field and set action to allow. Thus, only form data which with your customs' email can be submitted.
So our plugin is capable to process your huge list well.