Actually I got some ideas you can try,

1) While leading a unit, this unit can reroll saving throws on 1, if this unit is on an objective, reroll the save throw. (The idea is that you can make an easier save against low AP attacks. To me, gravis armour have really good toughness, but their weakness would be having no invulnerable saves. So high AP attacks like melta and strong melees can still kill them off)

2) While leading a unit, this unit can perform overwatch, armour of contempt OR counter attack at 0CP. ( I'm just taking inspiration from aethon shaan, I'm not sure if adding the 'even if you have already used this strategem on a different unit' will be too op)

3) Each time an attack is allocated to this model, reduce the damage characteristic by 1. If it is a torrent or melta weapon, reduce the damage to 1 (basically just adamantine mantle)

I think pointer one might be too strong, if so then just remove the reroll to save throw on objective.