We have added this feature in the most recent version of GridWeb. Please download and use the following fix for your needs.
It has added the following three methods relating to comments.
Adds GridCell.CreateComment method
Creates a comment object for a cell.
Adds GridCell.RemoveComment method
Removes the comment object of the cell.
Adds GridCell.GetComment method
Gets comment object on this cell.
Besides, the recent version can also show the existing comments inside the Excel file.