Read somewhere that if a unit that isn't in their territory already is gifted to them, it'll take 3 turns to appear within their borders. Not sure about that though.
It always takes three turns for gifted units to arrive at the designated city state. That's what it says on the tool tip, that's what I've observed in game. Doesn't matter where the unit is. In fact, I'm 99% sure that I've gifted a unit given to me back to the same city state it came from and it still took three days for them to get it.