You may face the following GPG error whenever you try to update the repository using apt update or apt-get update command
It could be fix the broken GPG signature validation for apt using rebuild the trust db.
Follow the steps to rebuild the trust db,
Remove or move trustdb.gpg, trusted.gpg.d (files and folder)
# rm -rf /etc/apt/trusted.gpg*
Then update the key using the following command
# apt-key update
Finally try to update the repository
# apt-get update
Still if you are facing the problem, you may refer the below link
https://gist.github.com/piersharding/ffc505564a82daaf6ff33d951f568dad
