Unless I wrote these incorrectly, here are the damage results:
>+6 0 Atk Sableye Foul Play vs. 0 HP / 4 Def Gyarados: 39-46 (11.7 - 13.8%) -- possible 8HKO
0- Atk Alakazam Foul Play vs. 0 HP / 0 Def Regigigas: 95-112 (26.3 - 31%) -- guaranteed 4HKO
So it's not going to do a lot, at least for the Sableye calc.
The reason for the Sableye one doing such a minuscule amount of damage is because it's level 1. Level plays a role in damage calculation, so despite Gyarados and thus Sableye having extremely high Attack, Sableye's level makes its attacks extremely weak. If we were to put is level back to 100, it'd do around 250% of Gyarados' HP, well over the OHKO.
For the Alakazam one, it doesn't do much since it isn't STAB, it has no item and also partially because Regigigas is naturally bulky with base 110 Defence.