Unfortunately, I tried that already – same results.
I think it's bricked. But how? If it has Dual Bios…
Can I send it to Gigabyte for Service?
Well you can ask gigabyte if they will replace the BIOS for you for a surcharge. However you will also have to pay shipping for sending your mobo to them. And back again (both ways). Unless you happen to live close enough to the Gigabyte RMA centre in your country / region whereby you could go there to drop it off in person. Assuming they are OK with doing it for you.
Another option would be to search for 'BIOS CHIP GA-Z170MX-GAMING 5'
on ebay. Those people are based in the netherlands. Its not that expensive and they include shipping. However you will also need to replace one of the 2 BIOS chips on your motherboard. Its not too hard but you need to have good-enough soldering skills.
Another thing you can buy yourself is a SOIC-8 flash clip. Those things can be wired into an rPi or similar device, to create a hardware flasher. And then use that to reflash your existing BIOS chips in-situ without having to desolder them.
In fact if you bought both products you could read what was on product A and then flash it onto product B without doing any soldering... and then make also a backup of it for later use with your hardware flasher should you ever end up bricking the BIOS again.
I would also research on why situations occur when both the BIOS chips get overwritten with a bad image / setting. It shouldnt really ever happen. Even if you have downgraded as you say (and only downgraded on one of them.
Maybe what normally happens is that on the 2nd successful boot of a new BIOS image, if its still good then it will automatically overwrite the other backup BIOS chip from what new version you must recently updated onto your primary chip. Not sure though. So you will have to look into it yourself. And as you say... its still not supposed to brick both of them.