Learn Italian: Mio marito è molto gentile.

Normally, Italian uses definite articles with possessives, like il mio gatto (my cat). However, there is a major exception: you drop the article for singular family members. Because marito (husband) is a singular family member, it is just mio marito.

What happens to the "no article" rule if I talk about multiple family members?

The article comes back! The exception is only for singular family members. If you were talking about your brothers, you would need the article: i miei fratelli.

Since marito is masculine, why does gentile end in -e instead of -o?

While many Italian adjectives end in -o for masculine and -a for feminine, a whole group of adjectives naturally ends in -e in the singular. Words like gentile (kind) or intelligente (smart) use the -e ending for both masculine and feminine nouns. So, you would also say mia moglie è molto gentile (my wife is very kind).

Does molto change its ending if it describes a feminine word instead?