I have a URL shortener/traffic analysis service hosted in a Google App Engine (GAE) project on Google Cloud (GC). When a URL is submitted for shortening, it performs a Python urllib2.urlopen to extract the page and get its title, which is uses to generate a social media post containing the shortened link. Some sites are returning a 403 Forbidden on the urllib2.urlopen command, and are unable to generate the body of the social media post from the page title as a result. Note that this only occurs when trying to load the page to get the page title; the shortened URL generated works correctly in my browser and 302s to the target page correctly.
My guess is that page requests coming from the GC IP ranges are blacklisted by the sites I'm trying to link to as a source of possible bot or other non-human or malicious traffic.
Does GC have a whitelisting service to which I could apply, or is this caused by something else?