gpg verification command is broken but this updated one should do the trick: Posted the new working instructions for adding the key but it must be templatized for ease of changes in the future:
scurl https://geti2p.net/_static/i2p-debian-repo.key.asc
gpg -n --import --import-options import-show i2p-debian-repo.key.asc
gpg --no-default-keyring --keyring ./i2p-pubkey.gpg --import i2p-debian-repo.key.asc
sudo cp i2p-pubkey.gpg /etc/apt/trusted.gpg.d/i2p-pubkey.gpg
sudo su -c “echo -e ‘deb http://deb.i2p2.de/ stretch main\ndeb-src http://deb.i2p2.de/ stretch main’ > /etc/apt/sources.list.d/i2p-release.list”