Definitely Heavy Slam
While Steelix is slow, Gyro Ball actually misses the KO on the two Pokémon you're trying to hit the hardest: Mega Gardevoir and Sylveon. True, it does well against fast Gardevoir, but a lot of them run only 20 Speed EVs and Trick Room. Here are some calcs:
0 Atk Mega Steelix Gyro Ball (61 BP) vs. 252 HP / 0 Def Mega Gardevoir: 116-140 (66.2 - 80%) -- guaranteed 2HKO
0 Atk Mega Steelix Heavy Slam (120 BP) vs. 252 HP / 0 Def Mega Gardevoir: 234-276 (133.7 - 157.7%) -- guaranteed OHKO
0 Atk Mega Steelix Gyro Ball (40 BP) vs. 252 HP / 0 Def Sylveon: 80-96 (39.6 - 47.5%) -- guaranteed 3HKO
0 Atk Mega Steelix Heavy Slam (120 BP) vs. 252 HP / 0 Def Sylveon: 234-276 (115.8 - 136.6%) -- guaranteed OHKO
Even with Sand Force Gyro Ball cannot KO. Either you run heavy Attack investment, or just run Heavy Slam.