My problem with this is that it's exactly how it was on SAMP. The claim was "they're highly regulated and scrutinised" and we all know how that turned out. I don't see any benefit whatsoever for legal gambling. It's just a business for someone to run, like any other. Illegal gambling, on the other hand, provides massive opportunities for illegal factions. Opportunities that do not exist when gambling is legal. Even if you have just one casino, why would you go to gamble illegally if you can just gamble there? It kills the industry for factions.
I'd like to see the opportunity for a tribal casino maybe, because at least that would be different and provide something to the server. It'd also reflect California's real gambling laws. Other than that, I don't see any reason why it shouldn't be illegal.
In future it'd be cool to have online gambling too, to balance out illegal gambling, considering that's where it's mostly done these days. I don't imagine developing an app like that is particularly high on the list of priorities at this early stage though.