To define wrongful gain:
1. Under Section 23 of the Indian Penal Code, 1860, wrongful gain is the gain by unlawful means of property to which the person is not legally entitled.
2. It excludes lawful gains or property the person is entitled to.
3. This concept is key in offenses like theft or misappropriation.
Therefore, the correct answer is (3) Gain by unlawful means of property which the person gaining is not entitled.