You're trying to buy one of the proper "Bundles" that exist on Steam, which are not simply a collection of items. When you buy such a bundle, you get a certain discount off the total price of the items in it, however items that you already own won't be counted.
Let's say there's a bundle with two items that cost $5 each, and the bundle itself offers a 10% discount. If you were to buy the bundle whilst not owning neither item in it, you'd have to pay $9 for it, ($5 + $5) * 90%.
Now if you owned one of the items, the base price of the bundle would drop to $5, meaning you'd have to pay $4.50 for the other item by itself.
As you can see, since the price depends on which items you already own, you can't purchase proper bundles to give away as a gift, since you'd only be buying the items that you don't own.