Gold dust IS help (he would not have stopped joki boy without it, he doesn't carry it himself and was just lucky it was available), so is Mizukage telling him the secrets of his clam genjutsu, Onoki explaining what joki boy does and Onoki destroying Mizukage's clam.
Anyway this battle Mizukage wins mid-high difficulty. Gaara won't get past the genjutsu without intel and even if he does Mizu's oil, joki boy and powerful water techs wins this for him.
the only secrest the mizukage told gaara abt the clam was that he needed to kill the real one,if the fought and the guy summoned a clam and suddenly it released a mist and there were illusions everywhere,wha would you think? Obviously it's the clam that's causing it,sand sensing finds the clam and then gaara destroys it,mizukage dosent have anything to get past gaara's sand,water gun isnt doing anything to gaara and what other water techs has he shown?,his oil is only used for his hydrification and joki boy,and pyramid has shown to easily overpower the hydrification.
Gold dust is a tool gaara used alongside the hail and his own sand,and he can easily select items to integrate into his sand,gold dust isnt the only thing that can cool things down,not to mention gaara can just select it and integrate it anyway,gaara figured out what to do with it,how to go about it and when to pull it off,he can do it again. His bushin feints fooled the mizukage twice,he can use that as well.