Time for some math.
A full set of i418(Forgiven's) (melee/ranged have the same mainstats, while casters have same main offensive stat but less VIT) gives 3003 STR & 3264 VIT.
A full set of i400(Aug Scavaen/Omega/Voeburt) gives 2851 STR & 3224 VIT.
Substats are much harder to gauge due to the fact that as you go up in levels, each point of substats devalues as the lvlmodifier increases, on top of the fact the gear's stats are all over the place so while one set may have a decent amount of crit, the next set up may have little to none of it, so I'll be talking strictly about the mainstats.
This means going from a full set of scaevaen to a full set of Forgiven's translates to roughly a 5% increase in your offensive mainstat, while giving you about 1% extra VIT(Yes, VIT on SHB gear does indeed scale that bad, the HP value increasing you see as you level is almost entirely the increasing scaling of how much each VIT is worth than any actual VIT you're getting). That's assuming you're also going from literally a full scaevaen set (so literally no other gear from 403-418) to a full set of 418 and nothing less, having even -some- non-400 or 418 pieces mixed in on either side further shrinks the miniscule gap. Skill infinitely matters more to damage/healing/tanking in Mt Gulg due to how small the gap is between Scaevaen & Forgiven's. For a lovely analogy to illustrate how small this gap is in the grand scheme of things:
Normal damage variance in this game is 5%.
TL: DR: Lrn2Rotation/Heal/Mitigate, gear ain't carrying you by any stretch of the imagination. If someone is taking tons more damage, doing bad dps, or is struggling to heal normal pulls, ask them if they need tips on how to play their job instead, since that's the actual issue.