When did the rainbow become a gay symbol?

So, Gilbert Baker was the first person to make the rainbow flag in 1978. The eight colors (which is now six, I believe) used were meant to represent different parts of people coming together.

Hot pink: Sex

Red: Life

Orange: Healing

Yellow: Sunlight

Green: Nature

Turquoise: Art/Magic

Indigo: Harmony/Peace

Violet: Spirit

One of the main reasons for its creation is that no one wanted to use the pink triangle which had been the closest thing to a symbol that they had (obviously it had bad backgrounds, due to the use by Nazis as an identifying symbol)