- I wonder why "not",as in "not quitting", has comma in front of "not"?
The writer was attempting to imitate a pause that would occur if someone were to read this in a dramatic way, sanctimonious way, or in a fashion of dispensing advice and speaking clearly as to be instructive or understood.
- whether is "quitting" present participle or gerund?
I'm leaning toward gerund because it looks like its renaming "difference" in the subject as though it was a predicate nominative.
- Do meaning of this sentence change if "not" is after "is"?
Yes. "W is not most often X" and "W is most often not X" mean two different things. The first negates the frequency "most often" and is saying "W is X" at a different interval. The second is saying "W is not X" at an frequency of "most often."
- Is "not quitting" reduced form of adjective clause?
No. You cannot stick which, that, or other relative pronoun anywhere in your example and make it work. You would need another verb. For example, you could say "The difference { that | which | that which } exists between winning and losing is not quitting."
-Is it possible that "not quitting" comes from "The difference which doesn't quit in winning & losing is most often"?
Invalid. A difference is not something that can quit in English, and you are saying the difference itself is quitting in your sentence above.