You could contact Gigabyte and ask them, though it wouldn’t surprise me if they just took the conservative approach and stated to use a lighter cooler without actually looking at any engineering data to qualify the statement, and it might not be realistic to expect them to test every board with every new cooler that hits the market, then there’s the variables of pre-existing stress on the board from a less than perfectly true case mount, even standoffs that weren’t completely screwed in.
I suppose one thing you could do is just order the cooler, have the motherboard out of the system and very carefully mount it, carefully hold the board propped up vertical on a table and use a straight edge aka ruler to see how much the board is flexing, then decide if you’re comfortable with that… but then if it breaks something doing that test, it wasn’t such a good idea. On the other hand if Gigabyte clears it as usable, document that, then if it breaks, at least you have that.
I still wonder if you *NEED* that massive a heatsink. If not, why fight an uphill battle? I also still assume it will work fine since Noctua lists it as compatible, though there can be some differences in rigidity from one board to a next, whether due to using a tighter fiber weave or other differences in the PCB material used, or proximity of adjacent rigid features using through-hole solder joints, like memory or other slots that will stiffen the board near them.
If you’re going to be doing mad-overclocking, then I’d also wonder about flex on the board potentially decreasing contact effectiveness of the heatsink on the row of ‘fets to the left of the CPU socket on many boards including the one you mentioned. An infrared heat gun could be used to track board temperature at various spots near the fets. Then again if that heatsink is screwed down instead of using spring loaded push pins, that will increase rigidity on that side of the socket as well and decrease any chance of flex causing reduced ‘fet-heatsink contact.
The easier answer would be to lay the system on it’s side or use a case that mounts the board horizontally, then put a rubber bumper (stick-on adhesive type placed where the case board mount backplane doesn’t have holes in it like so many do now) of the right height, directly under the center of the socket or strategically placed around the area to take up the extra weight and prevent board flexing. Again this probably isn’t necessary, just to take the sink off when transporting the system. I would do that with any vertically mounted (typical) heatpipe based ‘sink, not just one that heavy. I mean if doing more transportation than carrying it by hand, room to room.