My dentist here in Canada has signs up that advertise that you get a free electric tooth brush if you leave a 5 star review on Google. I've not done that, it seems quite wrong, but I would imagine this is also very much against Google's policies? I was surprised how clear and obvious the signage was. Seems to have worked though, the dentist's office has lots of 5 star reviews.
This sign always has the opposite desired effect on me. If I see a sign trading for 5-Star reviews, there isn’t any way for me to tell which reviews were written for a free burrito. So I immediately discredit all 5-star reviews the restaurant receives.
Obv not a perfect solution and unfair to those who aren't gaming but over the top 5 star and bitchy one star reviews even when genuine aren't always that representative.