Yes you can.

Like all things you buy on the Steam Store, everything (except for timed subscription licenses, such as EVE Online subscriptions*) can be stored into the inventory.

For pre-orders, the "pre-order" does not register until the gift of said pre-ordered item is binded to an account.
Basically, you can pre-order something for someone else as a gift. It is just that usual restrictions apply (such as "You can't play this until X date; when it's released").

^* Even EVE starter packs are not giftable.

Ok, I found the answer, and it's a weird one.

Basically, it was Time Machine's fault.

I use TM as my backup mechanism, but I back my system to an external HD. So, I naturally assumed that my backups were all exclusively on that external HD.

Nope.

Time Machine takes liberties with the free space on your system partition. It uses as much free space as it thinks it can to store a local backup, known as "snapshots". It does this as a safety net measure, in case you need to back your system up and you don't have access to the external hard drive where the main backup is maintained.

What happened (I'm not 100% sure, but I think it's a good enough guess to personally accept it as an answer) is that, when I deleted large files such as Borderlands 2, CS:GO, and even iPhoto (good riddance), Time Machine saw more available space for its snapshots, and used it accordingly. As deleted from here, Time Machine used the space there.

And here was I, thinking my SSD was broken because it was reporting my free space wrong.

Anyway, how to "fix" it?

By disabling this weird and obscure (though possibly helpful) Time Machine behavior. This article explains it in detail, but basically you just type sudo tmutil disablelocal into the Terminal and approve the command using your system password. This disables the snapshots feature, and you're left with only the external HD backups — as I thought was the case all along. (And yes, the article says Mac OS X Lion, but it's the same on Mavericks or Yosemite.)

That's it. I'm potentially less backed up now in case of an emergency, but now I have more available disk space than I know what to do with.

Guess I'm redownloading CS:GO...