Can a gas fireplace insert heat an entire house?
There is no way that a simple fireplace insert can heat a house the same way that a central air system that pipes the heated air into each room. 

It is possible to have a fireplace built that has a heat exchanger built into it that feeds into an air circulation system such that the warmed air is moved around to other parts of the house. However I doubt that your existing fireplace has this heat exchanger system built into it...then yet the ducting to an air curculation system into the rest of the house. 

I'm glad to see that you seem to have it in mind that the advice that you get from the fireplace salesman will always be biased toward what he is trying to sell you!!