You must not understand that there is no assurance that the gold bar really exists unless it is in the vault of the custodian, HSBC.
HSBC uses subcustodians. The Custodian is not liable for the acts or omissions of its subcustodians. There is no certainty that the gold supposedly owned by GLD really exists. After all, if there was complete certainty that the gold did exist, the objective of GLD would be to provide investors with the opportunity to own gold bullion by investing in shares of an ETF, rather than its stated objective to just track the price of gold.