For being such a pivotal moment in the story, there is not much description of the Garden of Death.  Sir Simon first mentions the Garden of Death in section five of the story.  At this point in the story, he is talking to Virginia Otis.  He is quite melancholy by this point.  He's fed up with trying to frighten the Otis family.  In fact, he's fed up with the Otis family pestering him.  Sir Simon is speaking with Virginia because he wants to ask her for her help.  He is seeking eternal rest, and Virginia can help him cross over to full death by bringing him to the Garden of Death.  

"They mean," he said, sadly, "that you must weep with me for my sins, because I have no tears, and pray with me for my soul, because I have no faith, and then, if you have always been sweet, and good, and gentle, the angel of death will have mercy on me."

Sir Simon tells Virginia that he will be able to finally sleep once he is in the Garden of Death.  He gives a beautiful description of the garden.  It sounds incredibly peaceful.  Long grasses and singing birds are specifically mentioned.  

"Far away beyond the pine-woods," he answered, in a low, dreamy voice, "there is a little garden. There the grass grows long and deep, there are the great white stars of the hemlock flower, there the nightingale sings all night long. All night long he sings, and the cold crystal moon looks down, and the yew-tree spreads out its giant arms over the sleepers."

Sir Simon also hints that time doesn't really exist in the garden either.  

"Yes, death. Death must be so beautiful. To lie in the soft brown earth, with the grasses waving above one's head, and listen to silence. To have no yesterday, and no to-morrow. To forget time, to forget life, to be at peace."

Whether there is or isn't time passing is irrelevant.  What's clear is that Sir Simon will get eternal peace and rest once in the garden.  That's it for narration about the Garden.  Virginia helps Sir Simon get there, but she never speaks about it with anybody.  Her husband even begs her to talk about it, but she kindly refuses.  

In Chapter Five of the "The Canterville Ghost," the ghost refers to the Garden of Death. According to his description, the garden is situated "far away beyond the pine woods," and has "soft brown earth" and long grass. The Garden of Death is silent, except for the sound of the nightingale which sings its song all through the night. The moon shines down on the people who dwell there and the yew tree protects them with its "giant arms."

The Garden of Death is a place of eternal rest. Inside, there is no concept of day or night, of today or tomorrow, nor of time itself. It is place in which the Canterville ghost will finally be able to sleep, after 300 years of being awake. But, to get there, the ghost must fulfil the prophecy that is written on the library window. He must enlist the help of a "golden girl" who will "give away" her tears and ask God for the ghost's forgiveness. The ghost chooses Virginia for such a task because she is the most gentle and kind member of the Otis family. To get to the Garden, Virginia must accompany the ghost and pass through the wall of the Tapestry Chamber, which they do at the end of the chapter.