Does google do anything to stop an app developer from just posting lots of 5 star reviews for themselves? Anyone can set up a gmail account and anyone logged into gmail account with their android phone or tablet can leave google play app ratings (with android 4.2 I can even have multiple google accounts for the same tablet)

So is there anything preventing someone from just creating lots of gmail accounts, then logging into their android device, leaving a 5 star rating for their app?
Short answer: No.

There is nothing that prevent developer from making lots of additional Google Accounts to provide 5 star ratings. Google recently made a change that require a Google+ enabled account to post reviews/star ratings on Google Play (at least using the on-device Play-app on certain phones) which is a small improvement from the past where a simple Gmail account would be enough.