The github repo is the right and official way to get in touch with the developers… and @donho is the only one who can do anything about the GPG key.

That said, if the GPG key expires, that just means that if you try to use GPG to verify the signature, it will complain about an expired signature. But there is nothing which will stop the Notepad++ application from working, even if the GPG key has expired. The GPG signature was just to give people confidence that the executable was what had been published, back in the brief period when Don didn’t have a license for the MS code signing; he subsequently got that license back, so MS checks during the installation process will still work fine.