Good morning,
Garrison artillery manned the big guns, usually mounted in the forts around Britain and the colonies. So places like Plymouth, Portsmouth, Chatham, Sydney harbour where the protective cover against the French, Spanish etc was situated to defend our dockyards.
They were also mobile but not in the same way as Horse or Field artillery. They needed huge teams of horses or trucks to move them around. When you see footage of very big guns firing in WW1 that's usually Garrison artillery. Can't do a link but google "Garrison artillery, western front, ww1".
John915