You can perform the calculation inline (not the best solution):

 <asp:TemplateField HeaderText="Actual Qty" HeaderStyle-Width="100px">
   <ItemTemplate>
     <%# Eval("ActualQuantity") == "0" ? "-" : Eval("ActualQuantity"); %>
   </ItemTemplate>
 </asp:TemplateField>

Setting up a formatting function is a better idea (not tested):

 <asp:TemplateField HeaderText="Actual Qty" HeaderStyle-Width="100px">
   <ItemTemplate>
     <%# FormatQuantity(Eval("ActualQuantity")); %>
   </ItemTemplate>
 </asp:TemplateField>

And in your code behind:

 public string FormatQuantity(string quantity)
 {
   return quantity == "0" ? "-" : quantity;
 }

If you need to display only, I would create a generic function that converts
all zero (0) values to NULLs and then you can utilize BoundColumn property
NullDisplayText="-". The function may look similar to the following:

public static void ConvertZeroFieldsToNulls(ref DataTable table, string
fieldNames){
if (table != null && fieldNames != null && fieldNames.Length != 0){
   foreach (DataRow row in table.Rows){
     foreach (string fieldName in fieldNames){
      if (!Convert.IsDbNull(row[fieldName]) && (int)row[fieldName] == 0){
        row[fieldName] = System.DbNull.Value;
      }
     }
   }
}
}