Just did another calculation of all the 696 Pokemon that resist or are immune (via Wonder Guard) to Steel, and I got 206. That means 490 Pokemon that are 1/5th or less MegaGross's weight take neutral or super effective damage from Heavy Slam. (Might be off since I may have counted Pokemon who have one type that resists Steel and another that's weak to it.) If it does get Heavy Slam, then it'll likely be banished to Ubers on Smogon XD (Though, I'd imagine even without it it may still be Uber.) However, since it can deal a lot of damage to a lot of Pokemon, then chances are it won't receive it.