The publication's latest disclosures showed Gulliver as the beneficial owner of Panama-registered Worcester Equities, which in 2007 had a balance of $7.6 million.
Though HSBC declined to comment on the story, Gulliver's legal representatives were quoted as saying that Gulliver "followed this procedure, because he wanted his taxed bonus earnings to remain private from his then colleagues in Hong Kong."
The lawyers, however, declined to answer why the chief executive's finances were held in a company registered in Panama.
"We must show we understand that the societies we serve expect more from us. We therefore offer our sincerest apologies," the letter read.
In response to The Guardian's revelations, HSBC said that in the the number of accounts in its Swiss bank dropped threefold in the last seven years, and the bank is cooperating with authorities investigating tax evasion.
However, the bank added that providing client data to foreign authorities is a criminal offense under Swiss laws, despite its intent to help the investigation.
HSBC is the world's second-largest bank and is headquartered in London. It has an international network of 6,200 offices in 74 countries, serving around 52 million customers.
HSBC has previously been charged with violating US sanction laws and laundering money to Mexican drug cartels. The issue was resolved in a landmark civil settlement in 2012.