Iterate through each Gridview row and find control and if found cast it to proper type.
foreach(GridViewRow row in grdViewName.Rows)
{
string studentId = row.Cells[0].Text.Trim();
if(row.FindControl("sessional1")!=null)
{
string sessional1= ((TextBox)row.FindControl("sessional1")).Text.Trim();
}
}