In essence it behaved very much like the sewers in terms of oneshotting, just in reverse.
Getting eaten alive by trash** in spite of running full deftard IB with close-to max*** block achieveable on RoR (++40% at the time), and similar avoidance otherwise, aswell as 0% ctbc etc was quite baffling, as was seeing party members die in an instant in spite of being well out of range of cleaves and not subject to NPC aggro.
It might be that these very issues have since been resolved, I haven't run Gunbad often since, as stated earlier.
In regards to backend, I don't want to be a prick and armchair-developer, but it might be reasonable to assign fixed threat-values across the board, so that changes to ability/class mechanic/tactic potency aren't messing with your aggro systems ~ that is if you haven't done so already in some form/it isn't like that by default (like the way taunts are handled by default as 2.5k crits or however it was).
** The very same trash, pulled the very same way and fought the very same way with but a slight change in group composition (= one dps), that behaved 'normally' in previous runs.
*** OT: WTT firstborn for t4 seals and blorc block tactics.