if you only want to called the gridview selected index change property on button click then just simply do one thing..
write below code on button click.
protected void Button2_Click(object sender, EventArgs e)
{
GridView1_SelectedIndexChanged(sender, e)
}
it just called the gridview1_selectedIndexchanged event in your button click event. then all your code written in the index changed event will fire. hope this answer helps you.