The game tracks your overall wins/losses in gambling. I had done some gambling before as part of the requirements for specific outfits and 100% completion but that was it. Still, I thought I'd won more than I lost and I figured I'd made over $1000 in winnings over the course of the game. I still figure I have, your losses cancel them out though. Last night I was something like $41 in the negative.
As people have said before, Five Finger Filet is the quickest and easiest way to make the money. It's $100 for about a 30 second investment. The patterns are easy to memorize and once you have them down you'll rarely miss once, let alone the 5 times it takes to fail out of it. Still, I've seen people saying they're like $3000 in the red due to gambling losses and that's gotta be tedious because that's like 40 rounds of FFF but that's what the requirement is. You can't just win $1000 in a game of high-stakes poker, you need to bring your entire total up past $1000.