No, he was not Nazi in the sense he cooperated with Hitler. He did not wear sieg-runes and swastikas on his robes. But he was a clear parallel to the Nazi and World War Two in the wizarding world. It is known Gellert Grindelwald embarked on a campaign of terror and muggle-racism several (about 8) years prior to his downfall at Dumbledore's hands in 1945, putting the war against him in exactly the same years as WW2 and the run-up to it, when Hitler threatened and slowly annected most of his neighbors until war was declared on him in 1939. Just the same, Grindelwald first threatened and picked up support before moving openly agains Muggles and opposition.
Also, if you have read Book VII, then the ideas and motives of Grindelwald ("for the greater good" & co.) are exactly the same as Hitler's in principle, only ported to a different environment. In this sense, Grindelwald is a Nazi, if you define Nazi as a power-hungry, deluded, racist fanatic.
Personally I consider this small, side-story parallel one of the finer points of the book, art-wise. (Yeah, I'm too old to enjoy it only as a fairy tale...)
And by the way, Voldemort's philosophy is Nazi too. Less obviously, but still very plainly, the central point of Voldemort's outward agenda is the superiority of not only wizards over muggles, but of a "pure" race of wizards over everyone else. Now where have we heard that before? Same as Hitler, Voldemort himself is not quite so pure. And same as him, Voldemort's unspoken ulterior motives aren't about race and nationalism, but about unlimited power and immortality for himself.
And that is in my opinion one of the strongest points of the books, that magical fairy-tale world dealing with the same monstrosities humanity had to deal with sixty years ago. Makes the whole wizarding world seem far more real, human and believeable all of a sudden.
On a related but unimportant note, it might be a fun idea to consider Grindelwald's campaign of terror to be the source of muggle conspiracy theories that Hitler was obsessed with the occult, and was practicing sorcery. Certainly if the German wizarding community has laws similar to the British Ministry of Magic, the German wizarding leader may have presented himself to Hitler when he came to power, just as the current British Minister for Magic reveals him or herself to the British Prime Minister of the day.