I saw you dancing
this neatly fits into the same syntactic pattern as German Ich sah dich tanzen. English has lost the infinitive inflection and subsumed it under a generalized -ing form that is a mix of gerund, participle and pseudo infinitive. This gives grammarians a nightmare. I'm inclined to call it wrong, because I hear you screaming is alright, I heard you scream is, but I watched you jerking off is utterly ambiguous. Yet, there is verbal aspect, and so see or watch command different aspects of time, as do hear or listen. So, in effect, dancing must be a present-participle. It cannot be a noun-like gerund: "? I saw you car"?
Growing things in the garden
as tchrist pointed out, there's a neat pun. However, there is no fully qualified clause, the phrase has no verb, so it's idiotic to talk about sentence positions or parts of speach. headlines use a specialized quirky grammar, after all, that strive for maximum generality without loss of specifity. In that sense it's entirely ambiguous. Given that Fear and Loathing in Las Vegas is acceptable, I'm inclined to say growing is a gerundive noun, which fits it's the title, and the the name of the book, a classical NP.
PS: I'm not sure, but I'd also try to reject the participle interpretation: , a passive past participle doesn't make much sense, grown things in the garden [I have]. However, the gerund still acts as a verb, that's its express purpose. compare "to be ernest" or "how to grow tomatoes" for a true or pseudo infinitive construction; is this "how" a relative pronoun? Is the whole phrase adverbial? I guess so.