The way I read that is that it will grant us GS while we’re channeling Divine Hymn. I don’t know what the raid mechanics are going to look like but it seems like it might be a viable workaround for not moving while channeling Divine Hymn. Not only will we heal ourselves 60% more with the Hymn but we can essentially eat a mechanic that would otherwise kill us while channeling (the Saladbar spinny mechanic from the current raid comes to mind). The second bullet point seems to explain what happens if we had already hard casted GS on ourselves prior to channeling Divine Hymn. That’s the way I read that anyway.
I like it thematically. I tend to stand in stuff a lot. This would let me do that more. It makes me feel good to not have to obey mechanics and not face reprocussions for planting my feet and pumping out healing.