I believe you are talking about purchases/payables and therefore that you mean GRN rather than GDN. As your vouching here must be to trace amounts ($) to the purchase ledger (presumably via a purchase invoice since it would be an invoice that is recorded in the ledger – not a GRN) this would be substantive. If you were checking that GRNs are matched with purchase invoices (irrespective of amount), this would be reperformance – i.e. ToC.