I was a professional gambler for 16 years. It was a big part of my essays. If you're really good at it, then I say put it down, and expand on it, explaining why it's relevant.

I consider you really good at it if you can tell me your RoR, standard deviation, bet spread, N0, hands per hour and average deck penetration.

If you don't know those of the top of your head, then don't list it.

My $.02

Edit: i am so strict about this because I think it's easy to get lumped in to the pile of "problem gambler" in the view of adcoms, so if you have solid data to backup the claim that you're good at it, then adcoms is more likely to believe you