I don't know much about Gel Fireplaces but what little I know, I'd have to compare the BTU output of Gel vs. Wood vs. Pellets vs. Coal vs. Propane vs. Natural Gas vs. Anything-else-you-can-burn-in-a-fireplace. At the same time, you also have to consider the 'material' (and how much you'd go through and how much you'd have to buy and how much it costs and how much effort it will take to clean up and so on.) PLUS, only a few of the ones I've mentioned can be VENTLESS.
It sounds like you're mainly interested in Ventless. Why so? You might want to look into the DOWNside(s) of a ventless system. Ask any Respiratory Therapist, or the like, about them. These systems can produce a lot of moisture, and in Winter where it's quite possible that the house may NEED moisture, there are rooms that will be COOLER than the one with the stove. And with this, it's quite possible that you'll have a moisture problem in them.
All I'm trying to say is shop around and ask questions. The Salesman you're talking to has a 'bias' towards what he's selling. Ask one Salesman about another's product and you may get some useful info. Visit the VENTED Fireplace Stores and ask them about Ventless Systems, and vice-versa. Now, about the resources ... See Ventless Fireplaces (DIY) and DIY Ventless Systems. There are other resources so don't conside my list as the end-all.