[geckoday]

If you want the include after the list of products you can check in the loop that lists the products. Change this:

{section name=product loop=$products}

To this:

{assign var="gwfound" value="n"}
{section name=product loop=$products}
{if $products[product].productid eq 3491}
  {assign var="gwfound" value="y"}
{/if}

Then after the {/section} wherever you want it add:

{if $gwfound eq "n"}
  {include file="customer/main/gift_wrap.tpl"}
{/if}

If you want it before the list of products you'll probably have to add a second {section} {/section} block to loop through the array just to check for it.