No, Glastrier isn't viable in RU.
Like Amethyst said, Glastrier isn't on the RU VR and is in the tier NU. I asked on Smogon if Glastrier is outclassed in RU and got the following response:
It's not a thing of being outclassed, more like "there's no place for Ice types in the tier outside of Snow. And even then it's a questionable choice".
There's not really a Pokemon that fills Glastier's role atm as a bulky swords dance breaker, but by virtue of being an Ice type it can't really do said breaking reliably against balance. You don't really do progress against defensive teams either, since most of them use either diancie (who can outbulk you and outdamage you with Diamond Storm's defense raises) or Quagsire (who can take a hit, Toxic you and recover stall). You could say that the Ice disadvantage is patched by Tera, but that makes it dead weight for most of the match. Weather being prevalent doesn't help matters either, since you're vulnerable to the 3 setters, only really taking advantage of snow (which again, is quite niche).
There's probably a team that can make it work (maybe with wish support and LOTS of hazard removal), but you're better off just using other physical attackers in the tier. If you really want to use an Ice type, use cloyster
Source
If you still want to use Glastrier, I'd recommend using Glastrier in NU. Glastrier was a good Pokemon in NU in the previous metagame at the time of this post, being A rank on the NU VR. (It's from the metagame before the December drops) I can't say exactly how good Glastrier is in NU at the moment due to NU receiving a lot of drops and having most NUBL Pokemon unbanned, but it still does have the things that made it good the previous meta. Glastrier's bulk, coverage moves, and Tera helps it break through offensive teams in NU, so it will still be useable there.
TL;DR Glastrier isn't good in RU due to the meta not being kind to Ice-types outside of Snow teams. Use Cloyster in RU if you want an Ice-type or use another physical wallbreaker in RU. If you still want to use Glastrier in a tier that can use its bulk, Tera, and coverage options, play Gen 9 NU.
