Gambling is a major sin which Allah has forbidden in His Book, and He mentions the prohibition on it alongside the prohibition of alcohol. Allah says (interpretation of the meaning):
“O you who believe! Intoxicants (all kinds of alcoholic drinks), and gambling, and Al‑Ansaab (stone altars for sacrifices to idols etc), and Al‑Azlaam (arrows for seeking luck or decision) are an abomination of Shaytaan’s (Satan’s) handiwork. So avoid (strictly all) that (abomination) in order that you may be successful.
91. Shaytaan (Satan) wants only to excite enmity and hatred between you with intoxicants (alcoholic drinks) and gambling, and hinder you from the remembrance of Allaah and from As‑Salaah (the prayer). So, will you not then abstain?”
When Allah forbids a thing, He also forbids helping with it in any way.
So it is not permissible to work in a casino because of what it involves of helping others in sin. Allah says (interpretation of the meaning):
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allaah. Verily, Allaah is Severe in punishment”
Whatever money a person earns by working in a casino is haraam wealth, and it is not permissible for him to make use of it or to spend it on himself. What he has to do is get rid of this money by spending it on various charitable causes.
You have to look for permissible means of earning halaal provision. If you leave this evil for the sake of Allah, Allah will compensate you with good. Allah says (interpretation of the meaning):
“And whosoever fears Allaah and keeps his duty to Him, He will make a way for him to get out (from every difficulty).
3. And He will provide him from (sources) he never could imagine”
And Allah knows best.