I got a Yubikey sent to me from Mt Gox for free. I think that was after one of the earlier hacks. Others who joined later had to pay for their key. In that case, having a secure key didn't help when the lock was smashed in.
Would it be possible to reuse my now worthless (other than Bitcoin-nostalgia collector value) Gox Yubikey for other purposes? I think I recall hearing they had a slightly different implementation of the standard.
I do not know what Yubi keys was distributed by Mt Gox, but generally you should be able to reuse it (I don't think I heard anything about some other hardware versions, that ones the Yubico are selling). If you are using Chrome browser, you can use it as U2F for example for Google and GitHub accounts. Other sites and other browser will join there soon.
Download and install YubiKey Personalisation Tool and you should be able to determine what hardware version of YubiKey you have.
And even more - with this software you should be able to program Steemit master password or better Posting Private Key (read why) on YubiKey! How about that :-).
I don't know. You could try setting it up with gmail, if it doesn't work maybe it's not the same as the U2F standard.