Was it a bad idea because she said "buy gas from Russia" or was it a bad idea because they only listened then ignored her advice?

What is this referring to?

Greta Thunberg

A strategy to not be dependent on fossil fuels due to climate change and a strategy to not be dependent on fossil fuels from Russia align very well.

I think GP might be referring to anti-nuclear movement

Germany shutting down nuclear plants in favor of natural gas plants and soon-to-be-increasing coal plants for base load is bad for energy independence, climate, and now world stability

That wasn’t Gretas demand

But she is anti-nuclear which IMO is very naive and unrealistic

Since you cannot change infrastructure over night, gas was the much cleaner alternative to lignite. If Russia stops supplying Europe, they will look for other gas suppliers.