Can't create giveaway for Borderlands: The Handsome Collection

is the whole thing just 1 key. thats all i can think of is its sold as a collection with more then 1 key.

You'll have to contact support and ask them to add it to the giveaway list.

Because its a bundle.

It may not be ideal, but for what it's worth, I've seen a few giveaways on here listing it as "Borderlands: The Pre-Sequel + Season Pass" with a further explanation in the description that it's the full Handsome Collection.

it exists on steam so why can't i make a giveaway for it here on SG?

To add to madsession's comment:
The only Steam store listing for it is a <Bundle>, and Bundles don't send out API data properly, meaning SG can't link to them. Keys for bundles are listed seperately in Steam's inner system, but as they don't have a separate store listing, there's no way to link to them.

Since the site only does manual listings for products that have been definitively removed from the Steam store, there's no present possibility for the Bundle being listed on SG. In the future, if the listing gets removed from Steam entirely (which'd only really likely happen if Gearbox introduces a new "complete collection" after additional game releases) or if Valve changes how the API works, then that may change. Given that the HJC is pretty recent, and that it's always a losing bet to count on Valve to update any part of their user-end features, neither of those possibilities is likely to occur any time soon, if ever.

As CallMeKap noted, the only possible approach for such listings is to post the giveaway under the next closest listing.
If you want to play it safe, have the winner comment with what version they received. If the full listing ever does get added to SG, all you have to do is link to that comment and staff'll update the giveaway listing for you at that time.