I just looked up the origin of “gangly”: it’s derived from “gangling”, which is an old word meaning “lanky”. Both “gang” and “gangling” have origins in old words that meant “go”. “Gang” came first, as it was a group of people who would go together, and “gangling” came from “gangrel”, which meant “a wandering beggar” [or “a beggar who goes about”]. Beggars that are able to travel from place to place tend to be tall, thin, and awkward, which is how “gangly” got its meaning.
Perhaps this is why “gangster” is sometimes used as an adjective for gangs.
