First off i'm going to assume intel wise - they only know what they saw each other do at the chuunin exams.
Gaara is going to know that chidori can overpower his sand, even if he makes a sand dome, so he'll try to take flight to make Sasuke a non factor, Deidara tried to do the same and guess what? It didn't work so well.
Sasuke has begun trying to blitz Gaara, while Gaara is beginning to take flight and sending sand attacks at Sasuke.
At this point Sasuke is likely to try and hit him with Chidori true spear like he tried to do to Deidara, I would assume that it's going to miss but it may hit, let's just say Gaara floats high enough to avoid it. Gaara will now know the range of chidori true spear, so he'll try to stay right outside of it.
Sasuke can simply pull out CM2, fly upwards and go in for a chidori hit. BUT WAIT! Gaara has just grabbed his limbs with sand, what shall Sasuke do now? Chidori stream takes care of the sand, Gaara is then killed and Sasuke goes home with a bounty on his head from the sand village :l.
Let's say Sasuke approaches Gaara's flight differently. He gets trapped by the sand, Gaara sand burials him. But Sasuke sneak attacks Gaara with a chidori the same way he did it to Deidara because Sasuke has Gaara put in a sharingan genjutsu to trick him, and since Deidara supposedly has a better trained eye, getting Gaara is even easier.
Let's say Sasuke exhausts all of his options, there is only one left which is Kirin.
Sasuke will pretend to shoot fire dragon at Gaara, when he's actually trying to hit the atmosphere. Leading to Kirin which Gaara has 0 defense against the best lightning ninjutsu in the Narutoverse. If regular chidori broke Gaara's defense, Kirin is doing the same.
I tend to think Sasuke wins this due to the Sasuke vs Deidara fight, Sasuke handled someone who's a better long range specialist than Gaara.
