I keep seeing people talk about how Gnb's mitigation is bad but looking at their personal options, it seems to me that the point behind their personal unique mitigation (Aurora/Camouflage/Brutal Shell) is to take care of the fluff mitigation, which frees up the heavier CDs combined with Heart of Stone to handle the busters.

Looking at it from a OT using their stuff on a co-tank, each tank is bringing a different way of handling the damage depending on the what the OT at the time is.

Drk- The Blackest Night - Provides pure raw mitigation, allowing the co-tank to us a minor/no cd to mitigate a TB.
Pld- Intervention/Cover - Provides flexible mitigation, allowing your co-tank to use a Rampart or some other mid level/no cd to mitigate a TB.
WAR- Nascent Flash - Provides effective HP that can be used to recover from/mitigate auto attacks and/or allow the use of a minor/no cd to mitigate a TB and recover the lost the lost HP.
GNB- Aurora/Heart of Light (honorable mentions to Brutal Shell)- Provides fluff mitigation to be used on their co-tank on cooldown allowing them to pool more of their personal mitigation to handle TB.

Each of these have their own distinct strengths and uses. If you try to use them to cover other situations that they may be ill-suited for, then of course they will underperform.