Europe was spending 200 billions / year on gas from russia. I imagine they could try to build 100 reactors for that price, but it would take a couple of years I imagine...
Gas is dispatchable. You can treat it like a huge battery. Nuclear power not only isnt a substitute for gas, it needs gas as a backup and to mediate supply and demand.
Gas is also waaaay cheaper than equivalent amounts of nuclear power - like 3x cheaper.
I suspect that you can modulate nuclear power too, but why do it? after you started the reactor it runs practically for free? (the fuel cost is so small; or it costs the same to run full power of half power). disclaimer: I did not read actual details about nuclear power plants designs in the past 20 years, so i'm vibing from first principles and bad memory
Gas is also waaaay cheaper than equivalent amounts of nuclear power - like 3x cheaper.