Question

What did the Gentleman's Agreement do?

Answer 1

It kept Blacks and Jews out of upper middle-class neighborhoods.

Explanation:

The "Gentlemen's Agreement" is an informal, off-the-books rider on any kind of business arrangement that is understood by both parties to be binding. In real estate deals, such arrangements were widespread (and appear to still be in force in many places). A common GA is to agree not to re-sell the property to Jews or nonwhites, or to resell it to anyone who would do such a thing.

There are other types of GAs in practice with less malicious and even more socially beneficial intents (like breweries agreeing to keep the alcohol level in beer below a certain point, or fashion houses not hiring excessively anorexic-looking models), but when a new player ignores what amounts to a tradition, it becomes largely unenforceable.