Gaité comes from the word gai which means happy. Gaité is like happiness. It's not very much used in the day to day language. I'm guessing that you're referencing the France Gall song which starts with "C'est comme une gaité, comme un sourire" which would translate as "It's like a happiness, like a smile". I wouldn't use it in a conversation. French talk and French writing are very different. It works in a song, but people would look at you in a strange way if you were to use this word in a convo :)