In theory, you could probably call Gigabyte and ask them to mail you the BIOS update on disk or CD or something (you know, the old fashioned way), and/or you might be able to tell them that you feel insecure with plain http, and maybe they'd change it for you...
But what you're saying points to a larger problem. How do you know that anything you download from any vendor (and that includes such hallowed things in the industry as Apple/Ubuntu/Red Hat/Microsoft/Google updates), is really secure?
The only way to get true security for anything is to build your own processor, build your own PC, write your own operating system, build your own network card, and then hope that there aren't any bugs...
Historically, things that were once thought to be secure -- have been proven over and over again not to be. Case in point: Windows NT -- it had labels all over the box, to the effect, "It's secure, it's secure". Well, fast forward 17 years or so. Numerous incidents and issues have historically proven those assertions to be in error... don't take my word for it... look at the history... Google "Windows NT security vulnerabilities" and you can also add the word "historical" in there, if you want.
That, and I'm pretty sure as a novice computer historian, that history repeats itself, although chances are that your BIOS might be perfectly safe even if you do download it with http (although, make no mistake about it, you are taking a chance, so "chance-taker beware", as the old saying goes...)
Computer security is a tough business, because on the one hand there's too little security, and on the other is outright paranoia... what's the correct balance between those two extremes? I sure as heck don't know...
Anyway... good luck with your BIOS update...