http://www.merriam-webster.com/dictionary/gaietyIn Beloved page 87 Paragraph 3

“She had felt warm satisfaction radiating from Beloved’s skin when she listened to her mother talk about the old days. But gaiety she had never seen.”

In this case I believe gaiety is the high spirits and merriment and lively quality of Beloved. This is a kind of happiness that Denver has never seen and this means a lot in a story talking about a time of slavery.