Bale is not a good fullback. At the start of his Tottenham career he was played as a fullback and of course he was quite average. But when played in his more favorable left wing position, he has been consistently good for Tottenham. Also lets not forget Giovinco is two years older than Bale, at this stage of their careers 2 years means a lot.
Also, if the question was, who was more fitting as a winger in a 4-4-2, i don't think there is a debate here, Bale is definitely a better option in that case.
As for who the better player is, i agree that we can't judge that until Giovinco gets an extended period of playing time.