try like this...
Protected Sub gridView_RowCommand(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewCommandEventArgs) Handles gridview.RowCommand
Dim gdvRow as GridViewRow = e.CommandSource.Parent.Parent
Dim txt As New TextBox
txt = CType(gdvRow.FindControl("textbox"), TextBox)
Dim Comment As String = txt.Text.ToString()
Response.Write(Comment)
RaiseEvent EventName(sender, e, Comment)
End Sub
hope this works...