The quickest way to go about buying lots of gift codes is to write your email address and credit card number (assuming you're paying by card) in a text editor or something, and then buy each game one at a time, copying your information from the text editor to avoid retyping everything over and over (and sending the gifts to your own email address because it's easier than looking up someone else's email address for each purchase).

Then visit this page to get a list of every gift code you have bought and PM them at your leisure.

You know, that many individual purchases would take forever. It's probably a good idea to contact support to see if they can make some sort of plan to handle all those gift codes from a single transaction (for example, maybe they can take one big gift and explode it into lots of little gift codes or something). I expect that they'll be eager to come up with a solution because it would save them a lot of transaction fees.