At your aspx.cs try below code:
foreach (GridViewRow gridrow in GridView1.Rows)
CheckBox cb = (CheckBox)gridrow.FindControl("MovieSelector");
if (cb != null && cb.Checked)
string obj = gridrow.Cells.Text;
totalId = totalId + ","+ obj;
this was you can work on every row for which check box is checkedand can update database with required details.
gridrow.Cells gives you the value of corresponding column in gridview.
Hope this helped !