This idea stems from the information of this thread,
The variable to preview the content of a private message is not available due to security concerns. Support did not share what the security concerns are and I am open to them being shared so Gainsight customers can assess.
For private communities, there does not seem to be a major security risk if private messages are previewed. Private communities typically have guardrails against bots and spammers and in an enterprise environment the risk of malicious content is minimal. The footer of the email notifications can also be updated to state that recipients should be cautious of opening links from senders they do not recognize.