1569461: 1745287: Vengeful Gladiator's Cleaver doesn't add to the ATK value of an ally, but rather replaces it.
Re-reading it, however, I can see it can be interpretable either way.
Your hero has dual wield. You can strike with Vengeful Gladiator's Cleaver while an ally in your party is in combat. (If you do, that ally gains this card's ATK and damage type this combat.)
No, you interpreted it wrongly. When an ally gains something, it's an addition, just like when they say "your hero gains protector". Then your hero has an added protector keyword.
I may be wrong though, we need clarification from the vets.
In the Comprehensive Rules 3.2 it says....
303.2c You can strike with only one weapon per combat (but see 406). However, you can strike with
that weapon multiple times if it can be readied during that combat.
If you strike with a
weapon multiple times, that wielder gains an additional +X ATK this combat from each
strike modifier, where X is the ATK of that weapon as that modifier was created.
303.2d
You can strike with some weapons “while an ally in your party is in combat.” Allies you
control are also wielders while you control such a weapon.
So the ally being the wielder of the cleaver like the hero....they gain an
additional +X ATK...their atack is not replaced.
I think this situation is similar to Illidan striking with a weapon. He gets 5 + the ATK of the weapon.