The GRE may ask you to associate a certain poem or author with the graveyard poets, but aside from that I wouldn’t spend too much time studying them. Just know what to look for so you can identify the genre and take an educated guess at the author. if you do see a  graveyard poem, chances are it was written by either  Thomas Gray or Robert Blair. Here’s an excerpt from Blair’s poem “The Grave,” which is pretty typical for the genre.

Robert Blair (1699-1746)

The Grave (excerpt)

While some affect the sun, and some the shade.
Some flee the city, some the hermitage;
Their aims as various, as the roads they take
In journeying thro’ life;–the task be mine,
To paint the gloomy horrors of the tomb;
Th’ appointed place of rendezvous, where all
These travellers meet.–Thy succours I implore,
Eternal King! whose potent arm sustains
The keys of Hell and Death.–The Grave, dread thing!
Men shiver when thou’rt named: Nature appall’d
Shakes off her wonted firmness.–Ah ! how dark
The long-extended realms, and rueful wastes!
Where nought but silence reigns, and night, dark night,
Dark as was chaos, ere the infant Sun
Was roll’d together, or had tried his beams
Athwart the gloom profound.

The Graveyard Poets include Thomas Parnell, Thomas Warton, Thomas Percy, Thomas Gray, James MacPherson, Robert Blair, William Collins, Thomas Chatterton, Mark Akenside, Joseph Warton, Henry Kirke White and Edward Young. James Thomson is also sometimes included as a graveyard poet.