No, you cannot apply for GST refund if you are leaving* Singapore via the Causeway as tourist refund is not available for land exits.
Currently, the Tourist Refund Scheme (TRS) is only available to tourists who are bringing their purchases out of Singapore via Changi International Airport or Seletar Airport subject to the eligibility and conditions of the scheme.
*WITHDRAWAL OF TAX REFUNDS FOR TOURISTS DEPARTING BY INTERNATIONAL CRUISES
Tourists departing by the cruise terminals (Marina Bay Cruise Centre Singapore or Singapore Cruise Center at Harbourfront) have until 31 August 2017 to claim refunds on purchases made before 1 July 2017. The eTRS facilities at the cruise terminals will be removed after 31 August 2017.