Would 'Storm clouds gathering' be considered as an example of personification?

Gather can be used in multiple ways:

1. [ no obj. ] come together; assemble or accumulate: a crowd gathered in the square.

2. [ with obj. ] bring together and take in from scattered places or sources: we have gathered the information.

   • pick up from the ground or a surface: they gathered up the dirty plates and cups.
   • ...
   • draw together or toward oneself: she gathered the child in her arms.

_{Oxford Dictionaries}

You seem to be thinking of the one with an object, which is not necessarily a personification ("somebody gathered storm clouds" and "storm clouds were gathered by someone" are not personifications). In your sentence, however, there is no object, which makes it a personification (since the clouds gathered themselves).