Dear Anh,
Noticed something while working on the test server. It seems that Gravity forms is not finding the customs fields which are set to use the custom table.
I'm not very experienced with how GF searches for custom fields. Maybe you need to find a hook to filter the list of custom fields (to add your own fields from custom table) or ask GF for help.
If Metabox Custom Table is writing it's own table separate of the User Meta, no, our add-on wouldn't integrate with that external table unless 1) the MetaBox Custom Table hooked into User Meta to 'intercept' the data query or 2) you wrote a custom hook to query that external table.
You might want to reach out to MetaBox support for assistance in the issue you're having and a possible workaround. Our add-on works with WP Standard User Meta which is in the wp_usermeta table (https://deliciousbrains.com/tour-wordpress-database/#wp_usermeta)
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