Hi Travis,
I've received the plugin GF User Registration, tested it on my local site, and found a way to solve this issue. After creating a custom field for users (or posts), the meta key (field ID) does not exist in the database while the User Meta in the User Registration plugin looks for an existing meta key in the database.
So, you can go to Admin Dashboard > Users > Edit a user > Update a value for the custom field, then back to User Registration Form > User Meta to select the user meta.