The Gau8 has effective armor penetration of about 80mm RHAe. [Penetration changes depending on velocity of the aircraft and dive angles.]
Modern tanks have 100mm+ RHAe in all directions, minimum. The front, especially the turret, is usually between 600mm and 1500mm RHAe.
The Gau8 is pretty much completely ineffective against any modern tank. You can kill old, unupgraded slavshit with that gun, barely, and crap like the M113 and all kinds of support vehicles and transports. But not modern tanks.
If you want to properly kill a tank with an A-10, you use a missile or a bomb.