- The comma before "not" is used to group "not quitting" instead of "is not." It's an odd way to use a comma, but necessarywithout it, this quote would be confusing.
- "Quitting" is most closely used as a gerund.
- Changing the sentence from "is 'not quitting'" to "'is not' quitting" changes the meaning from "not quitting is the difference between winning and losing" to "quitting is not the difference between winning and losing," which is a complete reversal!
- "Not quitting" is a gerund, and as such, is treated as a noun.
- The writer most likely used "not quitting" to emphasize that "not quitting" makes one a "winner," rather than emphasizing that "quitting" making one a "loser."