When you place a limit order, your order may not fill immediately if there are no opposite matching order on the market. And the asset in the open order is Locked, which is not available to trade or withdraw.
If the order keeps Open for long time, you may want to cancel it to release the locked asset to available balance. You can access Open Orders page to cancel it.
Wallet> Open Orders (https://gate.io/myaccount/myorders).