Is it really required to update Gigabyte GA-990FXA-UD3's BIOS in order to use an FX6300 CPU on this MOBO?

Most new MB's will get flashed at the factory to a later BIOS. The best thing to do is to look on the site and find out what BIOS version they added support for the FX6300 in, then either go t the store and physically buy the MB and examine it for the BIOS revision sticker on it to see if it's equal to or higher than that one needed to support the chip, or if ordering online, request that you need one with BIOS xxx.xxx or higher.

Some high end MB's can flash the BIOS without a CPU direct from USB but there is a rare function and most would need a compatbile CPU to boot up and flash.