hi,
First u need to get the gridview control id using find control,
Gridview gvChild=((Gridview)HeaderCheckGv.Rows[i].Cells[0].FindControl("ChildCheckGv"));
then u get the checkbox id using gvChild,
objdc.NameofFormCHK+=((CheckBox)gvChild.Rows[i].Cells[0].FindControl("GeneralPolicyCb")).checked;