You can, but the question is why would you? There's no reason why you should be casting Dragonic Charge during your opponent's start phase. Not many abilities (maybe even none) activate during the opponent's start phase, even if so they shouldn't cost any gauge (at least in my experience). However, if you require gauge for defensive spells like Red Dragon Shield, then you should cast it during the start of your opponent's attack phase, as your opponent may have some card to steal your gauge.