Looping through the grid is not a very efficient way to do this.
The best thing to do is to use the GroupByRowDescriptionMask property.
If that is not flexible enough for what you need, then you should use the InitializeGroupByRow event of the grid and set the Description there, instead of looping. You can determine the grouped column using the Column property of the GroupByRow. Some other userful properties on the GroupByRow are Value, Description, and DescriptionWithSummaries.