Gambling, like money, is neither inherently good nor inherently bad.

Any time one takes a risk on anything one can be said to be gambling.

Whether it is wise or unwise depends on the circumstances, and on the consequences of a loss. Risking a small sum of one's own money can be a reasonable form of entertainment, no more unethical than going to a movie, or eating at a restaurant. However, gambling the rent money is clearly unwise.

The unauthorized gambling of money or possessions that one is keeping safe for another person is not only unwise, it is a betrayal of trust. Even if one were to win the gamble, one could lose the trust.