Although most everyone says that cats purr not only when they are happy but when they are ill or frightened, I have to wonder. Are they working on a self soothing solution? Think about how the sound of a cat's purr makes you feel. As a human I find it inordinately comforting and it makes me feel warm and fuzzy and like I can just sit and nap all day or something. In other words, I am soothed.
Cats do purr when they are happy. I have to wonder if those who are purring in stressful situations (I once had a cat who wouldn't stop purring at the vet and she was holding his paws under water to try and make him stop and he just purred louder) are actually doing their best to soothe themselves.
I have no idea about this as I'm not a scientist, yet the part of me that humanizes my cats reactions kind of has to think so.
However, I'm sure that Gemini's soft little purr is simply because she's just such a happy cat in her world.