This is the classic pre-fight standoff – making yourself look bigger – defensive mode. We know kittens do this quite a lot and Gabriel is no different. What is, perhaps, a bit different is that he is doing it with his own shadow.
He does it a lot in the morning on the bed. It is a bit dark because the shadow on the wall is cast by a reading lamp, which I use when reading books while preparing some of the articles.
Kitten play, as we know, is all about hunting and fighting. It is preparation for that. It happens instinctively.
Play fighting comes instinctively to humans too. Look at all the hugely popular war video games which are becoming increasingly realistic. It is a bit worrying. For a cat it is simply about preparation for survival. For people it is about something quite different.