While Telenav is not the software that does what you want specifically, it can however, as a real time GPS unit, tell you the closest gas stations, sort by brand or by PRICE or both. Most GPS units are great but I haven't found one that carries the feature of telling you gas prices. In terms of how accurate this information is, I am not sure. Hopefully someone here can tell you that.
Telenav not only gives you directions to it but also tells you how far it is from your relative position (or any position you key in) and the full address and phone number hwich you can dial straight from it. There's a one month free trial if and I think it's something worth looking into for your needs.
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-wL
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