but gaara has his gourd sand sand from the sand box extra sand made fron the ground. He can use a descent amount of sand to block the fireball with his gourd sand while using the sand from the sand box to attack from above with his rain of sand,fire dosent really burn sand,he can attack and defend at the same time,also since itachi has some intel on gaara,he will know that getting close will be troublesome,as well as getting past his auto defence,since he is an edo,he has no chakra issues,so he will probably think of something.
However gaara also knows of his genjutsu capabilities as well,i dont think there is much water in a park to use his water dragon tech is there?,if gaara goes into his sand sphere which he can use his gourd sand alone for,then use the sand in the sandbox to attack,it will be difficult for itachi to get close,gaara can reuse his sand if he misses itachi,since it is only 3T,itachi can't really break out of a sand coffin/burial. If itachi decides to shushin,it might be a bad idea,as gaara's sand blocked enton/amaterasu(not sure which one anymorexd))three times consecutively quite easily from a rather close range position,he might get blocked,and if he does he is in big trouble,however if he manages to genjutsu gaara,he better pick a paralysis genjutsu,cuz the tricking type wont wotk as his sand protects him from self harm as well,if he manages to hit gaara with a paralysis genjutsu,he would get a chance to finish gaara up,but would he be able to kill him?,his defencce works while he sleepsxd,it will still be blocking his attacks,or wrap gaara in a sand shield. Normal 3t genjutsu isnt really as destructive as ms ones,gaara might be able to break free,since his handseal is a 4,which iirc also includes chakra control,if he can't,then it's basically going to be an annoying match for itachixdxd