Feel free to correct me if I'm wrong, but I can't think of an exception to this rule: I believe the difference between "increases healing magic" vs "increases healing actions" is the difference between incoming vs outgoing heals. All of the abilities that only increase GCD heal potency are buffs that apply to the healer, while the ones that universally increase healing are the ones that boost healing on the person healed.

Not that this isn't a weird and arbitrary distinction, but I think that's why those buffs are "allowed" to boost oGCDs. Because the buff can't tell the difference between incoming heal sources, so it just boosts all of them, whereas boosts like Temperance and Dissipation only affect GCDs because they apply to the person casting the heals.