76F1A20FF987672F is an identifying code number for both the public and private key that is associated with the releases stored in this APT repository. It is not a complete key - neither public nor private - and it is useless by itself.
The normal thing to do with one of these code numbers is feed it to
gpg --recv-keys to load the complete public key into your local key ring, but this particular public key isn't on the usual "key servers". There are instructions on https://wiki.winehq.org/Ubuntu for how to get it:
wget -nc https://dl.winehq.org/wine-builds/winehq.key
sudo apt-key add winehq.key
Running both of those commands should make
apt-get update happy again.
Running just the first command will give you a file containing the complete public key corresponding to the identifying code number. You can learn something about its contents with this command:
$ gpg --list-packets < winehq.key | less
The interesting part of the output is right at the beginning:
# off=0 ctb=99 tag=6 hlen=3 plen=397
:public key packet:
version 4, algo 1, created 1544460984, expires 0
pkey: [3072 bits]
pkey: [17 bits]
# off=400 ctb=b4 tag=13 hlen=2 plen=39
:user ID packet: "WineHQ packages <email@example.com>"
The "keyid" is the same identifying code number, and the "user ID" is an email address associated with WineHQ. However, don't take that for granted -- whoever generated this key could have set the "user ID" to anything at all. The normal way to determine whether a PGP key belongs to the person or organization you think it does is with the "web of trust", but this key isn't in the web of trust at all, so we have to rely on the fact that we got it from an HTTPS website belonging to the Wine project. This is probably good enough.
"created 1544460984" tells you when the key was created, but in an unhelpful way: that number is a count of seconds since the Unix epoch. You can turn it into something human-readable with the
$ date --date='@1544460984'
Mon Dec 10 11:56:24 EST 2018
It was created just ten days ago (as of when I'm writing this). This is probably why you were getting errors from APT -- they changed their key quite recently. This is a suspicious thing to have happen, but there's a note on https://wiki.winehq.org/Ubuntu saying that they did change their key, so it's probably legit, unless the entire
winehq.org site has been compromised.
The raw contents of
winehq.key look like this:
-----BEGIN PGP PUBLIC KEY BLOCK-----
[48 more lines of base64]
-----END PGP PUBLIC KEY BLOCK-----
You can see that this is much larger than the code number. For comparison, a PGP secret key looks something like this. It's even bigger.
-----BEGIN PGP PRIVATE KEY BLOCK-----
[76 more lines of base64]
-----END PGP PRIVATE KEY BLOCK-----
(That's a key I created just for the sake of writing this answer, never used to sign or encrypt anything, and immediately destroyed, even though you probably can't do anything interesting if you only have the first 96 bits of a PGP secret key.)