Use official databases for gas prices
In many countries (Spain, France, Germany, Italy, Croatia and others I guess) there are governmental databases about gas prices. They're official, reliable and updated daily or very frequently.
I know that Waze is supposed to be a social app, but I think that it is pointless to rely on users where this kind of official data is available, at least in Europe gas prices tend to be outdated and misleading.
There are many apps that allow you to search for the cheapest gas stations based on this databases, so it is definitely doable.