It wasn’t just „German Greens” who opposed buying Russian gas, it was a nearly universal EU ban.
It's kind of unclear just how much this organization actually is Greenpeace or not. That being said, extremist political parties and movements (e.g., UKIP), are so often financed by Russia as a form of covert ops that I wouldn't be surprised to find that they were a significant backer of Green Peace proper as well.