Are you saying that gas turbines don't burn fossil fuels?

No, I'm saying that using a gas generator as a backup for solar is better than not using solar at all.

They can burn non-fossil fuels.

A non-carbon containing fuel that is almost entirely created by burning fossil fuels is effectively the same (and often worse) than just using the fossil fuels directly.

Biodiesel or biomethane looks at first glance to escape this, but it has serious other costs.

Is almost entirely fossil-fuel created doesn't mean it has to be almost entirely fossil-fuel created. You know, just like observing that cars are almost entirely burning fossil fuels doesn't mean cars must burn fossil fuels.