I had a look at the report and it looks fishy. They're not actually comparing the cost of gas plants with the cost of wind, solar and storage - they're comparing with a hypothetical "clean energy portfolio" consisting of whatever combination of energy efficiency improvements, demand shifting, wind, solar and storage their model reckons is cheapest, effectively modelling energy conservation as a source of electricity that replaces gas power plant outputs. This makes very little sense; there's nothing inherent about efficiency improvements or demand flexibility that restricts them to being used with renewables rather than gas power.
That's not the only dodgy thing either. Their proposal generates substantial excess energy throughout most of the year. In order to improve its cost effectiveness, they assume that excess energy has a value of $15/KWh and subtract that income from the cost of the renewable program. The actual value of this energy is probably close to zero; remember, we're talking about energy that has gone unused even after substantial use of demand shifting and storage.