workaround is to use --nogpg.

1) this is not a bug for distribution-gpg-keys. This package just ships various gpg keys and does not handle any interaction with dnf or koji.
2) This error happened because you are on F26 and you are installing package from rawhide, which is signed using F28 key.
3) the file /etc/yum.repos.d/fedora-rawhide.repo contains:
  gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-$releasever-$basearch
and when you run:
  sudo dnf update kernel --enablerepo=rawhide
your releasever is still 26, so dnf this repo is configured to use the wrong key.
So you should either use 
  sudo dnf update kernel --enablerepo=rawhide --releasever=28
4) This does not happen in past because rawhide was not signed. It is signed now.

I think that the $releasver with gpgkey in fedora-rawhide.repo should be expanded. I.e.;
  gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-28-$basearch
But that has some negative side too (higher chance that .rpmnew file is created)
So I will leave it up to the Denis whether he will close it as NOTABUG or change the repo file.

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I think the same bug is still here. Rawhide branched and I am no longer able to upgrade my rawhide container. There should be a way to keep rawhide instance always up-to-date. I do not know how to do that now.

It does not help to use --releasever=31

# dnf list fedora-gpg-keys
Last metadata expiration check: 0:00:14 ago on Tue Aug 20 14:48:47 2019.
Installed Packages
fedora-gpg-keys.noarch                                              31-0.2                                              @rawhide
Available Packages
fedora-gpg-keys.noarch                                              32-0.2                                              rawhide 

# dnf repolist
Last metadata expiration check: 0:18:04 ago on Tue Aug 20 14:48:47 2019.
repo id                       repo name                                                                                   status
*rawhide                      Fedora - Rawhide - Developmental packages for the next Fedora release                       59328
*rawhide-modular              Fedora - Modular Rawhide - Developmental packages for the next Fedora release                   0

# dnf upgrade --enablerepo=updates -y fedora-gpg-keys
Last metadata expiration check: 0:19:01 ago on Tue Aug 20 14:48:47 2019.
Dependencies resolved.
================================================================================================================================
 Package                                  Architecture              Version                    Repository                  Size
================================================================================================================================
Upgrading:
 fedora-gpg-keys                          noarch                    32-0.2                     rawhide                    100 k
 fedora-release                           noarch                    32-0.1                     rawhide                     11 k
 fedora-release-common                    noarch                    32-0.1                     rawhide                     19 k
 fedora-repos                             noarch                    32-0.2                     rawhide                    9.5 k
 fedora-repos-rawhide                     noarch                    32-0.2                     rawhide                    8.6 k

Transaction Summary
================================================================================================================================
Upgrade  5 Packages

Total size: 149 k
Downloading Packages:
[SKIPPED] fedora-gpg-keys-32-0.2.noarch.rpm: Already downloaded                                                                
[SKIPPED] fedora-release-32-0.1.noarch.rpm: Already downloaded                                                                 
[SKIPPED] fedora-release-common-32-0.1.noarch.rpm: Already downloaded                                                          
[SKIPPED] fedora-repos-32-0.2.noarch.rpm: Already downloaded                                                                   
[SKIPPED] fedora-repos-rawhide-32-0.2.noarch.rpm: Already downloaded                                                           
warning: /var/cache/dnf/rawhide-2d95c80a1fa0a67d/packages/fedora-gpg-keys-32-0.2.noarch.rpm: Header V3 RSA/SHA256 Signature, key ID 12c944d0: NOKEY
Fedora - Rawhide - Developmental packages for the next Fedora release                           1.6 MB/s | 1.6 kB     00:00    
GPG key at file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-31-x86_64 (0x3C3359C4) is already installed
The GPG keys listed for the "Fedora - Rawhide - Developmental packages for the next Fedora release" repository are already installed but they are not correct for this package.
Check that the correct key URLs are configured for this repository.. Failing package is: fedora-gpg-keys-32-0.2.noarch
 GPG Keys are configured as: file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-31-x86_64
Public key for fedora-release-32-0.1.noarch.rpm is not installed. Failing package is: fedora-release-32-0.1.noarch
 GPG Keys are configured as: file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-31-x86_64
Public key for fedora-release-common-32-0.1.noarch.rpm is not installed. Failing package is: fedora-release-common-32-0.1.noarch
 GPG Keys are configured as: file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-31-x86_64
Public key for fedora-repos-32-0.2.noarch.rpm is not installed. Failing package is: fedora-repos-32-0.2.noarch
 GPG Keys are configured as: file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-31-x86_64
Public key for fedora-repos-rawhide-32-0.2.noarch.rpm is not installed. Failing package is: fedora-repos-rawhide-32-0.2.noarch
 GPG Keys are configured as: file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-31-x86_64
The downloaded packages were saved in cache until the next successful transaction.
You can remove cached packages by executing 'dnf clean packages'.
Error: GPG check FAILED

Current issue can be fixed by commands:

# dnf upgrade  https://kojipkgs.fedoraproject.org//packages/fedora-repos/31/0.5/noarch/fedora-{gpg-keys,repos,repos-rawhide}-31-0.5.noarch.rpm
# dnf --releasever 32 upgrade

I could not find signed 31-0.5 packages in any update, so secure upgrade is broken by these steps.

rpm -qpi fedora-gpg-keys*.rpm reveals no key.

(In reply to Petr Menšík from comment #6)
> Current issue can be fixed by commands:
> 
> # dnf upgrade 
> https://kojipkgs.fedoraproject.org//packages/fedora-repos/31/0.5/noarch/
> fedora-{gpg-keys,repos,repos-rawhide}-31-0.5.noarch.rpm
> # dnf --releasever 32 upgrade
> 
> I could not find signed 31-0.5 packages in any update, so secure upgrade is
> broken by these steps.
> 
> rpm -qpi fedora-gpg-keys*.rpm reveals no key.

That is because we do not have a completely fedora 31 compose yet. It doesn't exist. we are working on it. 

If we had that everything would have worked, but we don't. 

Perhaps next branching we should look at disabling rawhide composes until branched works/pushes out, but thats a big hammer.

I have issue with installing new kernel from rawhide in Fedora 32 (after upgrade from 30 and 31 not sure if it's important).

I'm installing with:

$ LC_ALL=C sudo dnf update kernel --enablerepo=rawhide --releasever=32
...
Downloading Packages:
[SKIPPED] kernel-5.7.0-1.fc33.x86_64.rpm: Already downloaded                                  
[SKIPPED] kernel-core-5.7.0-1.fc33.x86_64.rpm: Already downloaded                             
[SKIPPED] kernel-modules-5.7.0-1.fc33.x86_64.rpm: Already downloaded                          
warning: /var/cache/dnf/rawhide-2d95c80a1fa0a67d/packages/kernel-5.7.0-1.fc33.x86_64.rpm: Header V3 RSA/SHA256 Signature, key ID 9570ff31: NOKEY
Fedora - Rawhide - Developmental packages for the next Fedora  1.6 MB/s | 1.6 kB     00:00    
GPG key at file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-32-x86_64 (0x12C944D0) is already installed
The GPG keys listed for the "Fedora - Rawhide - Developmental packages for the next Fedora release" repository are already installed but they are not correct for this package.
Check that the correct key URLs are configured for this repository.. Failing package is: kernel-5.7.0-1.fc33.x86_64
 GPG Keys are configured as: file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-32-x86_64
Public key for kernel-core-5.7.0-1.fc33.x86_64.rpm is not installed. Failing package is: kernel-core-5.7.0-1.fc33.x86_64
 GPG Keys are configured as: file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-32-x86_64
Public key for kernel-modules-5.7.0-1.fc33.x86_64.rpm is not installed. Failing package is: kernel-modules-5.7.0-1.fc33.x86_64
 GPG Keys are configured as: file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-32-x86_64
The downloaded packages were saved in cache until the next successful transaction.
You can remove cached packages by executing 'dnf clean packages'.
Error: GPG check FAILED