In general, nuclear power and renewables mix quite poorly. This is especially relevant for replacing gas, since gas is great at providing variable power (when needed) which neither nuclear power nor (most) renewables are good at.
You can also see this move to more variable gas generation by looking at the increased gas power plant install base (
https://www.energy-charts.info/charts/installed_power/chart....) without the corresponding increase in generation.
I wouldn't overrate the amount of gas that is freed by nuclear power (also do keep in mind that there is also quite a lot of heat-electricity cogeneration with gas in Germany, which further complicates matters).
If Germany only didn't shut of any nuclear power plants prematurely, then emissions could be reduced significantly since coal would probably be directly replaced. But I doubt the expansion of renewables in Germany would have happened as it did without a parallel nuclear exit in the 2000s. Would it have changed much if the exit in the 2010s didn't happen in terms of renewables? I don't know. But I also think a non-exit would have been politically untenable after Fukushima, with pretty much every party simultaneously being suddenly gung-ho in shutting down nuclear.
I think the phase-out of nuclear, could everything else have stayed the same, was a missed chance. But at the same time western countries which held much more strongly onto nuclear, especially France, are very likely seeing a huge electricity generation issue in the next decades due to very low rates of building nuclear (with huge difficulties) or renewables (not much interest) and a large amount of nuclear power plants going of the grid (https://de.wikipedia.org/wiki/Liste_der_Kernkraftwerke_in_Fr..., the list in the englisch wikipedia unfortunately doesn't include planned closure dates). These dates could (and will) be extended, but as we're seeing right now they're already getting unreliable and nearing the end-of-life.