what you miss is what row is selected.
So
Dim ddl As DropDownList = DirectCast(sender, DropDownList)
Dim gRow__1 As GridViewRow = DirectCast(ddl.NamingContainer, GridViewRow)
Dim myGrid As GridView = DirectCast(grow.NamingContainer, GridView)
myGrid.SelectedIndex = gRow__1.RowIndex
Now the code nows to see what row is needed.