The quote is from a paper called "How to Combat Hitlerism", which tells you something. Here's the full quote:
I doubt if the Germans of future generations will entertain unadulterated pride in the deeds for which Hitlerism will be deemed responsible. They will honour Herr Hitler as genius, as a brave man, a matchless organizer and much more. But I should hope that the Germans of the future will have learnt the art of discrimination even about their heroes. Anyway I think it will be allowed that all the blood that has been spilled by Hitler has added not a millionth part of an inch to the world’s moral stature.
It's in the context of a discussion of non-violent resistance, and discusses what might have happened had the Czechs and Poles employed it rather than meeting violence with violence. It's certainly not expressing general admiration for Hitler.
IfIt should also be pointed out that this dates from June 1940, at which time Gandhi would have had no knowledge of Hitler's intentions for genocide.