The definition of an adjective is a part of speech that is used to describe a noun. Red, tall, better and fast are all examples of adjectives.
In modern parlance, a gentleman (from gentle + man, translating the Old French gentilz hom) is any man of good, courteous conduct. A gentleman may also simply be a polite term for any man, as in indications of gender-separated facilities, or as a sign of the speaker's own courtesy when addressing others. The modern female equivalent is lady.
Here officer is understood to include commissioned officers, cadets, and midshipmen of both sexes, hence the more common term conduct unbecoming. A gentleman is understood to have a duty to avoid dishonest acts, displays of indecency, lawlessness, dealing unfairly, indecorum, injustice, or acts of cruelty.