Mm...theoretically, I guess so, but the chances are you'll be misinterpreted, I think. What do you really mean by saying it? If you mean you'll go buy a watch, I would have said "vou comprar", and if you mean you'll find a watch in your closet, maybe "vou achar".
Even for the English "I'll get you a taxi", I think I wouldn't have used 'ganhar' although it might work out. I'd probably say "vou procurar" or "vou achar" in this case.
(I'm not native -- others might correct me!)