As the writer and director (not to mention an uncredited dog-walker in one scene), Morrisette crafted a near-perfect modernization of the Scottish play.
As an adaptation, Gnomeo & Juliet is a little loose with the original, though it does hit the major points of the play up to when the movie says it’s deviating.
Loathe as I am to criticise Shakespeare, I'm not sure this play is really all that well-balanced, although I'm not familiar with the text, so I'm not sure how much of it was adaptation.