HDB Loan doesn't accept Credit Card payment directly, at least not that I know of. So, same for GrabPay MasterCard.
Alternatively, for AXS payment, you can pay with normal banks' Credit Cards (EXCL GPMC), via AXS Pay+Earn @ 2.5% Fee. GPMC is blocked for AXS payment.
Another option for HDB Loan payment, you can pay using Credit Card (incl GPMC), via iPayMY or CardUp (usual fee @ 2.25%, current promo, lowest @ 1.6%). However, GPMC doesn't get Reward Point for such payment. Several other banks' Credit Cards do award Cashback or Reward Point instead.