DBNull does not implement the == and != operators, although can use DBNull.Value.Equals(object). The preferred method is Convert.IsDBNull(object) as chobo used.
For strongly typed DataSets however, this is not the correct approach, as you will get these errors. When you create the DataSet, the rows will create an IsField
Null() method for each nullable column. So for AchievedMark the row will have a method bool IsAchievedMarkNull(). For each field then you could do
gridViewCells = row.IsAchievedMarkNull() ? String.Empty : row.AchievedMark.ToString();
The other option is to use the untyped row.IsNull("AchievedMark"), but that kinda defeats the point of creating a typed DataSet.
PS. Could you possibly add line breaks to the last line of code, so it doesn't stretch off screen and make things hard to read. Thanks