Ok, I think I figured it out.
Adding a hidden field and setting {date_dmy} as default value will work for all new entries, but not for the existing entries.
Since Gravity forms already has a date created field for each entry, there is no need to st it again (via the default value). Instead, leave the default value blank and use the same approach as for displaying the entry id (as shown in the FAQ section of the plugin page).
How can I add the entry date to the list?
Add a (hidden) field to your form and note the ID of that field and then add this code to your functions.php
add_filter('filter_entries','add_entry_date' );
function add_entry_date($entries) {
foreach ($entries as &$entry) {
$entry["xxx"] = $entry["date_created"];
}
return $entries;
}
Change xxx in the code above to the ID of your new field.