You can cancel withdrawal orders before they are confirmed on the blockchain. However, the orders whose confirmation is done are not revocable.
This is how to cancel withdrawal orders.
www.gdac.com/exchange -> Deposits/Withdrawal -> transaction history -> cancel
Withdrawal orders are not always confirmed on the blockchain network. In that case, coins are taken back to GDAC wallet, so please ask for the coin refunds through the link below.
Smart Help Desk will give you answers within 2~3 days but Answers are often subject to delays.