In other words, you want to know how to attract people typically searching casino or something related to casino (on Google Ads) without you bidding for keyword(s) "casino", etc.
This requires in-depth keywords research and usually no one will give out their hens which lay their golden eggs.
Nonetheless, some people are a bit generous enough to provide some hints(s) for you to build up on :-
For example, in the real brick and mortar world, usually where are casinos located (in which type of establishment) ? Think of that and use any available creativity (if any) to build up from there. Ofcourse, there are many others white keywords that can be used. I provided 1 clear hint, creativity can result in getting more keywords.
Do not forget, that even if using white keywords, if you need good conversions for casino campaigns, it is better to cloak the white page and about this aspect, it has already been discussed to the fullest possible extent (whatever can be openly shared), in various threads in the https://www.blackhatworld.com/forums/google-ads.83/ section.