The symptom of this issue is seeing "Send TEXT_GV_NAME To Customers" as the title for the page shown by Admin->Discounts-Mail Gift Certificates.
(Other pages in Admin-Discounts have the same problem of GV_NAME not being set in their language strings.)

The new order of inclusion of language files in 1.5.6 Admin (page-specific file, extra_includes, main language file) means that the GV_NAME variables can no longer be defined in english.php. I migrated them to a separate file and pull that file in where it is required.

https://github.com/zencart/zencart/pull/2089