He's not the first champion to have a VGU while also having a legendary skin; Tristana was the first, but in Tristana's case it was a super early legendary skin that only had new spell effects, and since Riot's stance on VGUs is generally to only bump up the quality and not to add new features (with notable exceptions like Pentakill Yorick and Karthus), Firefighter Tristana is effectively has the same content as it always had, which is significantly lower than legendary skins are touted for.
Meanwhile Galio's legendary skin was among the first if not THE first to be an almost complete overhaul of everything the base champion had at the time; new VO, new animations, new VFX, new SFX, etc. He was also the first skin to have a login screen made for him if memory serves. Given that Gatekeeper Galio was made ages ago he lacks some animations that newer champions and skins have, such as a recall, homeguards run, return to base animations, and even then he's lacking because he still has some of base Galio's animations such as the same joke, laugh and dance animations and no taunt.
My personal hope is that the VGU team on Galio says "Sod it, let's make it a proper legendary" and give him new stuff to replace what he's missing, because I feel like Gatekeeper Galio's close enough in terms of quality to modern legendaries that he should get that bumped up. Hell depending on how good and usable base Galio's animations are they might even get to do the Dunkmaster Darius and Mecha Zero Sion path and leave some already great animations in while still going the extra mile on quality.
So Reav3, I don't know what the team's plans are, but I'd say it'll be very much worth the time if you guys go and...I guess "complete" Gatekeeper Galio with his upcoming VGU. I feel like it deserves to be worth its cost given how strong the theme is, and this is the perfect time to do it. I mean you guys are gonna have to give him a new VO, animations and VFX for the skin anyway to be consistent with what's on live right now so why not just add what's missing to make it of the quality of modern legendary skins?