The amount of HP required to push the gargoyles back or have them stagnate seems to vary. It may differ based on which step towards the bell they are on (first having a higher threshold of damage needed to prevent them from moving forward vs when they're right next to the bell it seems to be about 10% will prevent them from ringing). But even these guesses were inconsistent - one attempt saw them ringing the bell despite getting to 85%. The best strategy seems to be to burn cooldowns on one gargoyle and kill it within the 20s window (since it heals itself to full when it moves). They have less than 1.5 million HP. The gargoyle didn't respawn, but I've heard it said that if the bell is rung all gargoyles will repop.
Our strategy once one gargoyle was dead was to spread our DPS across each of the remaining three and call for rotations when the gargoyle in any given wing was one step away from the bell. The gargoyles can be tanked/healed by one well-geared tank and one cleric CHealing once they're slowed. Send one cleric and one tank to each wing along with a handful of DPS and have some folks designated as floaters who can move to whatever area is lacking in DPS the most.
The gargoyles seem to be belly casters - meaning you need to stand DIRECTLY underneath them for most spells to land.
Once 15 minutes pass, the 4 enforcers become active. The gargoyles no longer need to be DPSed, they will stand near the bell but not ring it. The enforcers will fling periodically but can also be single tank/healed (bomb heals worked better than trying to CHeal) once they are slowed. I believe there is a 10 minute timer on this phase, but they should be focus fired since there's no real benefit to balancing them. The enforcers also cast a debuff that turns you into a gargoyle and will result in a DT if not cured.