As far as I know, you can offer gift wrapping as a separate listing, so long as you are prepared to sell JUST the wrapping paper on it's own. Which is strange I know, but otherwise the listing would be against the TOU (you aren't allowed to sell an item that must be purchased with another). Check out this helpful blog post for how to do it right :)
Highlights on how to do it right:
•The gift wrap should be a stand-alone listing in your shop. The buyer must not be required to make a separate purchase from your shop in order to buy the gift wrap.
•List the gift wrap as a commercial supply if you’re using paper and packaging you’ve purchased. The top-tag (Category) should be Supplies; the second tag should be Commercial. Be sure to also tag it as gift wrap.
•If the gift wrap is handmade by you, you may list it as a handmade supply or paper good. In this case, the top tag (Category) could be either Supplies or Paper Goods. Be sure to also tag it as gift wrap.
•Be prepared to sell only the gift wrap if someone purchases it without also buying an item from your shop. (You’re selling the gift wrap as an item, not the service of wrapping. It’s possible a shopper might just want the wrapping paper.)
•Be sure to include the details about this gift wrap (dimensions, materials, colors, embellishments, etc).
•Make a note in your shop’s item descriptions that you offer gift wrap, and the buyer must purchase the gift wrap in addition to the item they would like wrapped.
•Also note this gift wrap will only be available for items in your shop. (Some new shoppers may not realize Etsy is made up of many unique shops.)
•You may want to make a note that your customer should purchase the item and the gift wrap at the same time. You can note this in your gift wrap listing and your item listings.
(from:
http://www.etsy.com/blog/en/2007/holiday-how-tos-sellers-wrap-it-up-with-gift-wrap/ )