Ok, lets break this down, the only Tonberry immune to Gravity is Don Tonberry.
Don Tonberry walks towards it's target to use it's attack Chef's Knife which has a chance to inflict death.
Sin can produce countless sinscales, unlimited if you take what happens at Zanarkand before Tidus and Auron knock down the exploding trailer, therefore all sin would need to do is summon an endless supply of scales and have them do the dirty work, no matter how many Don kills, they would eventually topple him.
According to the FF Wiki "Sin is immune to most conventional form of weaponry and is highly resistant to attack, able to project a barrier to protect itself. It is believed in-game that only the Final Aeon can kill Sin, and all attacks the party wages on Sin in Final Fantasy X only serve to wound it — it is not until the death of Yu Yevon's spirit that Sin actually dies." Meaning that if Don Tonberry wanted a chance to kill Sin it would need to be inside Sin, like the party is when they fight Jecht and Yu Yevon.
Lastly Don does not have an airship to be able to get inside Sin.
Conclusion: Sin uses sinscales to topple the mighty but landlocked Don. Don - who can't attack Sin kills countless scales but can't reach Sin (like anyone who uses non ranged weaponry).