I want my Contrary Shuckle to have Shell Smash and Gyro Ball, but inverted Shell Smash would give Shuckle -2Atk, -2SpAtk and -2Spd and +1Def and +1Spdef. But will Gyro Ball's damage be lowered by the -2Atk? I've gone on Battle Spot to test this and Gyro Ball did an extremely bad amount of damage. The speed that my Shuckle is at right now is 15 at level 100.
Of course it does.
Gyro ball inflicts more damage the slower the user is compared to the target, calculated by the following formula:
Base Power = 25 × (Target's Current Speed / User's Current Speed)
But what you get is only the Base Power of the move, not the effective damage you will do. If you have a -2 stage on attack, Gyro Ball will be half its normal power.