They're not updating visuals for individual skins. Skin updates will only occur with VGUs.
Galio is so far going to be the first champion who has a remotely advanced legendary skin who's getting updated. All the old ones like for Annie, Trist, TF, etc weren't true legendary skins because they were ancient, ancient relics from a time when a new particles effect was considered 1820 work. Galio actually has a different model, a different VO, and different particles in Gatekeeper, and it's not so outdated that it's not retired, making it the first mildly advanced skin to get updated in a VGU.
At present, Rioters have said that they are actually investing a lot of effort into Gatekeeper to make it worthy of it's price tag, and giving it a unique new VO as well as other things. We'll see how involved they are with Gatekeeper, and use this as a gauge for how they'll approach future VGU champs with high cost skins.