Jerrod6: you could choose to measure power in Watts or BTU/h.  Similarly you can measure energy consumption in kWh or BTU regardless of whether we're talking about gas dryers, electric dryers, light bulbs, water heaters, cooktops etc.

In order to compare dryers, it's best to choose one system and stick with it -- in this case, power in kW and consumption in kWh.

Gas dryers are not more efficient than electric dryers.  In fact, they actually use slightly more energy due to the combustion process and its moisture being drawn through the drying chamber.   But of course gas is cheaper and typically emits fewer greenhouse gasses even when compared to heat pump dryers depending on the energy mix in your region.  

So Energy Star figures might show something like this:

Electricy cost: 14 cents per kWh

Gas cost: 4 cents per kWh

Electric vented dryer: annual consumption 500 kWh costing $70 carbon footprint of 100 kg

Electric heat-pump dryer: annual consumption 250 kWh costing $35 carbon footprint of 50 kg

Gas dryer: annual consumption 600 kWh costing $24 carbon footprint of 40 kg

Same here Jerrod. 

It's telling you that they used 658 or 685 kWh of gas.  Would you prefer this be in cubic feet, therms  or BTU?  Electricity could be measured like that too if you prefer.  But to compare between dryers you need to pick a single system and use it for both.  So the dryers you're referring to use 658 kWh and 685 kWh of gas.

When I lived in Phoenix, AZ from memory the cost of my utilities was as follows:

Natural gas: 4 cents per kWh

Electricity: 6 cents per kWh off-peak (weekends, holidays and weekdays 7pm through midday)

Electricity 22 cents per kWh winter peak-rate (weekdays midday through 7pm)

Electricity 25 cents per kWh winter peak-rate

So if a dryer used 658 kWh of gas, that would cost me 658 x 4 = $26

If an electric dryer used 658 then it would obviously depend on when I was using it.  But could be well over $100 if I often used it during the peak rates.