I'm sorry to burst the bubble, but this will NOT work as wanted. Why?
1: The setup Gorebyss must achieve
The Gorebyss needs to be insanely lucky to get a Substitute AND both of its boosting moves with its very average bulk of 55 / 105 / 75. Almost any Electric- or Grass-type special attacks, or even powerful physical attacks granted they get a boost will hamper the strategy. Moreover, Gorebyss will be as passive as a slug trying to outspeed a cheetah while setting up. Why? Because while setting up, your opponent can prepare themselves to fight the receiver. Taunt? Whirlwind? Hazards? Toxic before the Sub? Very common strategies will disrupt the setup with no efforts. See this, as well as that.
2: The very nature of Regis golems
Those 3 are weak to Fighting-type, one of the most common types. In addition, 2 of them are weak to Ground ; 2 are weak to Fire, and 2 are weak to Steel. If the plan was to use them altogether, then there would be no synergy at all. But even then, none of the 3 would synergize with the rest of the team. They also have pathetic offensive stats and Assault Vest prevents them to boost except through Power-Up Punch. Even then, as replays demonstrate, the attack should be supereffective to do anything significant. So the strategy is a big no actually, sorry.
Now, to properly answer your question,
If your friend is adamant on using this strategy albeit pretty bad, then Registeel is his best bet.
Registeel @ Assault Vest
Ability: Clear Body
EVs: 252 HP / 4 Atk / 252 Def
- Iron Head
- Power-Up Punch
TL;DR: the very nature of this strategy is pretty gimmicky and will not work.