First Click Free on Pantheon
Google demotes/bans results from publishers who show different content to web crawlers and people. They also demote/ban publishers from having content in Google News if it does not show ALL the article content.
For publishers who want to require registration for full-content access, Google has instituted a "First Click Free" policy. Simply put, if you let people coming from Google domains see all content, then Google's ok with you. Note this is only for their first page view that was referred from a Google domain; subsequent pages they view can show truncated content.
Resources
Dynamic Cache: Allows you to skip Drupal's page cache for a given page (as you probably surmised, will not skip Varnish).
https://www.drupal.org/node/322104: D7-relevant thread, recommends dynamic cache as being the codified solution, looks towards D8 to solve this.
https://www.drupal.org/node/875152: D6-relevant thread, which outlines the approach that was used to create Dynamic Cache.