For anyone stumbling upon the same problem, do not count on the help of WPDM crew.
I am disappointed with the support I got (or I actually didn’t get).
Anyway, if you want one of the hidden fields of Gravity Forms to be populated by Package Name, you have to go to:
wp-content/plugins/wpdm-form-lock/wpdm-form-lock.php
and add the following lines of code in line 149:
$wpdmpro_name = get_the_title($pid);
$formhtml = str_replace("[wpdm_package_name]", $wpdmpro_name, $formhtml);
Good luck