I don't believe there is a DSM for MDD.

From what I can tell the difference between MDD and regular daydreaming is the addictiveness. There is some anecdotal evidence that it may be sometimes treatable with SSRIs. I spoke to someone whose career is psychiatry and they felt that it was part of Fantasy Prone personality. https://en.wikipedia.org/wiki/Fantasy_prone_personality

I think with gifted kids that it is natural based on being a child and having a vivid imagination that they have these types of experiences, without it being MDD. However, it could also be Non-Autistic Motor Stereotypy - which has a much better prognosis and a similar description.

MDD's main characteristic appears to be the addictiveness of the fantasies (most prefer their fantasy worlds to reality, so much so that they remain in their worlds the majority of the time to where it affects day to day life on an often profound scale).