The Gravity Forms Advanced Post Creation Add-on doesn’t appear to use ACF’s API functions in order to save post meta on newly created posts. It’s likely using core meta functions such as
update_post_meta() instead of ACF’s
update_field() and in doing so, the ACF Custom Database Tables plugin doesn’t have a chance to intercept the call and store the data in the appropriate custom database table.
In order to make the plugins work together, it’s necessary to hook into the post creation process and save the field values using ACF’s update_field() function. This will allow ACF Custom Database Tables to locate the appropriate custom DB table and store data there.
An example of this is provided in the below snippets and is based on the Gravity Forms documented approach here.
Given the Gravity Forms Advanced Post Creation Add-on is using core meta functions, ACF Custom Database Tables won’t be able to properly bypass core meta storage when configured to do so.
Using ACF Forms as a simpler alternative
For a more cohesive approach, consider using the Advanced Forms Pro plugin for front end forms that create/edit posts. It uses ACF’s API functions correctly and is compatible with ACF Custom Database Tables.