It is probably possible to achieve it with functionExternalModules: true, requre() and global.get(), but maybe there's a more convinient way inside the editor.
Also, I'm not sure how functionExternalModules works with the new versions of Node-RED.
You'll probably see it contains the OAuth2Client function/class meaning you can access it via googleAuthLibrary.OAuth2Client or const {OAuth2Client} = googleAuthLibrary