High power to size (weight) efficiency, very fast start up/shutdown and you can run them on a variety of fuels, quite cheap (relatively to other fossil fuel plants, fuel varies on where you live) - if you use the waste heat (since they produce a lot) to produce steam, you get quite an efficient cycle.

They scale up as well, most of our (UK) fossil fuel for energy is Combined Cycle Gas Turbine's (since we took coal completely off the grid a few years ago).