Nuclear isn't directly an alternative to gas power. You've got the problem stated right in your comment: the constant output of nuclear is only part of generation needs.
Gas is different from nuclear in that it's easy to regulate the power generated, which is necessary to deal with demand spikes and generation dips (the latter due to wind/solar which may not always be available). Presumably Germany would only ever want to rely on gas for this purpose.
There are ways to adjust nuclear power output but they are quite expensive (on top of the fact that any nuclear power plant is already expensive).
To get rid of fossil fuel power plants you need to match their exceptional ability to regulate power generation levels. You could do this by using nuclear for base generation and supplementing renewables with storage techniques (e.g. pumped hydro storage), but nuclear is unfortunately not a "drop in" replacement on its own.
The situation you’re describing arrives from a status quo of lots of renewables and no nuclear. The actual starting point was nuclear and no renewables. If they had both renewables and nuclear they could have done a number of other things with their energy design.
Nuclear energy is bad at providing flexible output and that's what is needed.
And with rising temperatures the exiting power plants aren't reliable at all because the the need cool water supply
