another interpretation is that griever is the messenger. but i can only back that up with relating these:
sleeps=griever is within squalls mind, as well as the rest of the party's minds, so in a way it is dormant until they fight ultimecia and she summons griever.
garden= there's two ways to approach this. griever is within squall, and squall is a SeeD for balamb garden, so griever is in balamb garden... or you can relate this to future esthars theory about ulti's castle being a version of a Garden like Balamb's and Galbaldias, so griever waits to be awaken in ulti's castle/garden.