I reckon that you know what a nominalized adjective is. But just in case you don't, it's an adjective acting as a noun. In the movie title - the good, the bad & the ugly - good, bad & ugly are nominalized adjectives. They are adjectives acting as nouns.
Der Alte means "the old man". How would one say "the old woman" (other than die alte Frau) or "the old one"? Can alte, or any German adjective for that matter, be any gender - masculine, feminine or neuter? Well... maybe not ALL German adjectives!
I know that German adds suffixes to nouns to change the gender. For example, Lehrer is teacher & Lehrerin is a female teacher.