Amazed you can trademark a slang word: https://en.wikipedia.org/wiki/Git_(slang)
If your "GitStart" startup was intended to help "unpleasant, silly, incompetent, annoying, senile, elderly or childish person[s]" to start doing something, I think you'd have no problems from a trademark perspective.
If your "GitStart" startup is to help people start doing something with Git, the (trademark-ed) source code management system, then you might be in trouble.
IANAL.
See https://git-scm.com/about/trademark for their policy on this.
However, I don't think it's the case, since the word GIT has been used by other entities long before GitHub came to be. Otherwise, we wouldn't have GitLab, for instance.