I think the GK should get this thing as a passive ability and the talent should buff it. Since he has no innate DR and no mobility or otherwise “get outta jail free” card, AoE damage from gas, fire and rattling is indeed your biggest curse.
Suggestion: Either give him flatout 50% DR against AoE damage and let the talent improve it to 75% or something, or make it work as his version of Gromril armour that only works against AoE damage:
After receiving AoE damage, GK becomes immune to AoE damage for 3 seconds. Talent buffs it to 5.