If you are using Item template in your gridview then try this:
string strID = ((Label)gvEmpDetails.Rows[0].Cells[0].FindControl("lblID")).Text;
string strName = ((TextBox)gvEmpDetails.Rows[0].Cells[1].FindControl("txtName")).Text;
string strAddress = ((TextBox)gvEmpDetails.Rows[0].Cells[2].FindControl("txtAddress")).Text;
If you are not using template field then write simply this:
string strID = ((Label)gvEmpDetails.Rows[0].Cells[0];
string strName = ((TextBox)gvEmpDetails.Rows[0].Cells[1];
string strAddress = ((TextBox)gvEmpDetails.Rows[0].Cells[2];