You forgot to redefine DataSource in your PageIndexChanging event.
Modify your code to:
private void BindGridView1()
{
try
{
this.GridView1.DataSource = ProjectManager.GetHoodListing();
this.GridView1.DataBind();
if(Convert.ToInt32(DropDownList1.SelectedValue) != null)
GridView1.PageSize = Convert.ToInt32(DropDownList1.SelectedValue);
}
catch (Exception ex)
{ throw ex; }
}
protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
GridView1.PageIndex = e.NewPageIndex;
BindGridView1();
}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
BindGridView1();
}