Sure it’s safe. That’s why there’s a dual BIOS switch. If you mess it up you can just switch the BIOS and it will still work. I would definitely recommend only trying it once though.

Is updating GPU BIOS safe?

Estimable. Yes. Be careful to follow manufacturer’s instructions when flashing bios, as it indeed is risky and can bork your windows installation (and the motherboard itself). Also, backup your stuff before you do anything and have your windows install usb key on hand in case you need to run a quick repair on boot.

What does it mean to flash a GPU?

“Flashing a graphics card” is just a short-hand way of describing the process of writing an image file (BIOS, UEFI, EFI, Open Firmware, etc.) to the card’s own “Flash” memory. Typically something like nVFlash is used to do the deed.