"Most"?
Healers main source of healing comes from oGCDs, healer gameplay revolves around doing the least amount GCD heals as possible, I'm pretty sure any guide will tell you that.
WHM is an exception as its Lilies are GCDs but not a DPS loss since they allow to build Afflatus Misery which at the minimum neutral on single target if you don't align it with party buffs.
GCD heals only exist if you run out of oGCDs.
