“It indicates on the DMOZ site that a category with fewer than 100 sites is a small category and a good category for a beginner to edit.”
Unfortunately Gambling is just one step up from Adult. There are different, unwritten rules. Also, that 100 sites includes sites waiting to be reviewed, which you have no way to know about. I doubt there are 60 Roulette sites waiting, but there could easily be a few dozen if the category has gotten a lot of spam submissions. Still the basic principle is if there is a subcategory that could be applied for, then do it. You can move up to the larger category in a week or less if you show you can do a good job.