https://www.eia.gov/analysis/studies/powerplants/capitalcost...The economics of the pure gas turbine vs the combined cycle depends on the fraction of the time it spends running.
For a peaking plant you are only going to run it a fraction of the time so you care about the capital cost and not about the fuel cost... So you're going to use the cheaper to build but more expensive to operate pure gas turbine.
If you are running the plant all day then the economics are favorable to go combined cycle where you start with a gas turbine, recover heat from the exhaust, and feed that through a steam turbine. The capital cost is more than the peaking plant, less than the steam turbine on it's own, but the fuel efficiency is great which leads to lower operating costs. (e.g. the steam turbine in a combined cycle plant is much smaller than a stand-alone steam turbine with the same energy output as the whole plant)
In the case of nuclear it is worse though because you do absurd things: a PWR has "steam generators" which are huge heat exchangers, larger than the pressure vessel, that use hot water at high pressure to boil water at low pressure. For liquid metal reactors the heat exchangers are even larger
https://www.youtube.com/watch?v=JIkYOFxcqRg
Contrast that to the heat recovery units used in the combined cycle turbine that couple hot gases to water or the sodium-to-air heat exchangers that rejected heat from the fast flux test facility. In terms of size these are like a bug splatter on the plans for a PWR.