In Soulsilver, what would be better for Forretress?
Well, seeing that Forretress only weighs 277.3 lbs (125.8 kg), and he has a Base 40 Speed stat, I would recommend Gyro Ball.
Gyro Ball would fit better to Foretress beacuse it's a very slow Pokemon.Heavy Slam, not so much beacuse there are a lot heavier Pokemon than Foretress and it isn't a good strategy though,plus Gyro ball deals more damage the slower the user is and Heavy Slam's damage doesn't depend on stats and as I said before there are a lot heavier Pokemon than Foretress.
Go with Gyro Ball.
HOPE I HELPED YOU
If asked me, I would prefer you using gyro ball
Due to Forretress's low Speed, Gyro Ball. Also, as Mew pointed out, Heavy Slam didn't exist in SS, so Gyro Ball is your only option.