Notice that when the sun shine, there is no need to use natural gas, so gas use is reduced. Your argument is mistaken.
You seem to have missed a very important data point I have already given. Combined cycle power is twice as efficient as gas turbines.
I'll give a simple example. Assume that whenever the sun shines the solar panel produces maximum power. Assume that when the sun does not shine that all power comes from gas turbines. Peak power usage is not at noon, but at about 6:00PM, when the sun sets. So about half of the energy used is produced by solar, half by natural gas. Now assume that there were no solar panels, or natural gas turbines, but instead only combined cycle natural gas. Combined cycle is twice as efficient as the gas turbines so for the same fuel we get twice the electrical energy. In both situations we burn the same amount of fuel and get the same amount of electricity. Because it takes a lot of material and labor to make a solar panel the energy it produces costs twice what that same energy would be from combined cycle. Because gas turbines burn twice as much fuel as combined cycle that energy costs twice as much. So, we get the same energy, burning the same amount of fuel, and pay twice as much for it. A total loser.
My very simple example ignores a lot of what happens in the real world, most notably that solar power produces the same output at noon as it does at twilight. Real world means that relying on solar mean increased carbon output or only very slight reductions. Either way it means a near double increase of cost in electricity.
I'll go one step further, just to twist the knife. Solar power is right now about 20% efficient for common utility grade panels. Theoretical efficiency for photovoltaic panels is near 60%. Theoretical efficiency for a solar power heat engine is 95%, current efficiency is about 30% with future expected efficiency near 75%. To be kind I'll give solar power a 3x gain on efficiency, and say that makes solar power cost 1/3 what it does now. Solar power now is estimated to cost between double and triple what coal, natural gas combined cycle, nuclear, and hydro, cost. Those four power sources are near equal in price. Being kind again I'll say the price is only double, and we will assume future manufacture costs remain the same so future solar would cost only 2/3rds what electricity costs now. A win then, right?
Not so much. Solar power only works during the day. We'd need something to not only store that solar power but produce enough excess to store for the night. Now we have to first double the solar power produced, which turns 2/3rds the cost to 4/3rds the cost. Then we need to pay for the storage mechanisms, since we are already at a loss I don't see a point to speculate that cost.
What if we don't store the energy? Use something else to make up for the night time? Like natural gas turbines? That gets back to what I laid out before, but instead of a double in cost we get something like 1.5x the cost. What if we use wind? Wind doesn't always blow and costs the same or more than the natural gas turbines. Going any further and we have a grid much like we have now except with perhaps solar contributing perhaps 15% to 20% rather than what we have now where it's less than 1%. Going any higher than that 15% and there is a risk of destabilizing the grid from severe weather and the odd solar eclipse.