http://replay.pokemonshowdown.com/gen7pu-733803052
I did a test in-game where I brought a level 30 Shuckle, holding Iron Ball and with minimum speed (7), to Melemele Meadow and ran into a wild L11 Cutiefly. The Cutiefly used a number of different moves, including a Stun Spore to get the paralyze, while Shuckle got to -6 speed (extra redundant, since it's being halved twice) and used Power Trick.
At that point, the calculator (and evidently the simulator too, based on the linked replay) thinks something like this is coming up:
252+ Atk Shuckle Gyro Ball (150 BP) vs. 0 HP / 0 Def Cutiefly: 792-932 (2400 - 2824.2%) -- guaranteed OHKO
Instead, the ensuing Gyro Ball dealt exactly
2 damage to Cutiefly (verified by throwing a Poke Ball at it and checking the post-capture summary screen, which revealed that it had 28/30 HP left). So evidently if you use Gyro Ball when your current modified speed is 0, the game is smart enough to avoid crashing for division by 0, but ends up rejecting the entire calculation up to that point and defaulting to 0 damage, which then gets adjusted up to the minimum 1 and then the 2 is probably a result of applying the type advantage.