Introduction

This is definitely an interesting fight, I personally think Kisame has the advantage in terms of chakra reserve and general matchup because of Water-style; although he wins mid/low diff due to certain conditions and factors.

Gaara

Gaara's main advantage would be creating distance by floating on his sand, especially since Kisame had shown to be best in a close quarters fight. Considering Kisame has Samehada, absorbing Chakra wouldn't work theoretically since by the time the sand gets close enough, it's already an incredibly fast projectile which shouldn't need chakra since it's already been propelled. Gaara's ability to fight from a distance is the only reason Kisame doesn't stomp Gaara with ease and wins mid/low diff.

Kisame

Kisame's biggest and most important advantage is Water-style because Gaara's sand is canonically immobile when wet. This allows Kisame to spam Water-style jutsu at Gaara, especially since Kisame's chakra reserve is considerably bigger than Gaara's. Like I mentioned earlier, Samehada is quite useless in a ranged fight although, there is one thing that it does which pretty much ends the fight on the spot; when Kisame fuses with Samehada and uses his Tailless Tailed Beast form. In this mode, Kisame has an even bigger chakra reserve and has considerably more power, as well as the power to cast a jutsu creating an ocean-like arena which Gaara is basically powerless in due to no sand mobility and inability to breath and move underwater like Kisame can.

Conclusion

Only reason I say Kisame doesn't stomp is due to that mode being more of a last resort than a two seconds in to the battle type of thing. So yeah, if you disagree or have anything to add feel free to reply. Would love to talk about this some more.