If you want to check in button click event then cod eshould be like this:-
for (int i = 0; i < GridView1.Rows.Count; i++)
{
CheckBox chk = (CheckBox)GridView1.Rows[i].Cells[0].FindControl("CheckBox1");
if (chk.Checked)
{
}
}
Here in Cells[index], your index is the position of your checkbox column location.
Hope you find your solution.
Fun coding....