Hello, Hitesh,
It could be a bit of a negative factor. Gaps in employment are not always negative, but often can be problematic. It depends on the reason. And preparing for the GMAT probably wouldn't be an impressive reason to adcoms. For one thing, even if you get a high score, the adcoms might handicap it, thinking that you had to take extreme measures to achieve it compared to others with a comparable score who earned while maintaining a regular schedule. Second, it deprives you of the ability to write engagingly about current involvements in work challenges. Third, it may make adcoms question your priorities - while they value the GMAT highly and place certain weight on it, they also don't like to see applicants who seem obsessed with it (fairly or not).
If the GMAT is the main reason and you haven't done it yet, you might want to rethink your plans for it. If you've already started your sabbatical, I suggest also getting involved in something noteworthy on a volunteer basis that links coherently with interests you'll discuss in your application and/or that have some relevance to your goals.
Also, it depends on what you mean by sabbatical - if you are taking a break from your work but will remain with your company and have their blessing, it would be better than if you were to leave the company and apply as unemployed.
Best regards,
Cindy Tokumitsu
Senior Editor,
www.Accepted.com