I did find out the instructions on that page have some errors. Still working internally to correct. You are correct in that the key file is wrong. The instructions on that page point to the deb key (which is binary) instead of the rpm key (which should be ascii).
In the meantime, i was able to at least install graylog-server on SUSE:
sudo rpm --import 'https://downloads.graylog.org/repo/el/stable/GPG-KEY-graylog'
sudo zypper addrepo --gpgcheck --refresh 'https://packages.graylog2.org/repo/el/stable/5.0/$basearch' 'graylog'
When attempting to install, zypper does return a “file from repository is unsigned” warning:
Retrieving repository 'graylog' metadata ----------------------------------------------------------------------------[\]
Warning: File 'repomd.xml' from repository 'graylog' is unsigned.
Note: Signing data enables the recipient to verify that no modifications occurred after the data
were signed. Accepting data with no, wrong or unknown signature can lead to a corrupted system
and in extreme cases even to a system compromise.
Note: File 'repomd.xml' is the repositories master index file. It ensures the integrity of the
whole repo.
Warning: We can't verify that no one meddled with this file, so it might not be trustworthy
anymore! You should not continue unless you know it's safe.
File 'repomd.xml' from repository 'graylog' is unsigned, continue? [yes/no] (no): yes
However, you can choose ‘yes’ at the prompt to continue. I understand this is not ideal and probably not recommended (to ignore signing) unless this is for testing or a lab of some sort.
We will work to resolve these issues. Thanks.