Michael H. Bright believes that Shelley relied on his understanding of Gilpin's picturesque while describing this Ionian isle:
In particular, one may single out William Gilpin, a foremost proponent of the theory, who designates in ''On Picturesque Travel'' the proper objects for the traveler's attention: ''But among all the objects of art, the picturesque eye is perhaps most inquisitive after the elegant relics of ancient architecture; the ruined tower, the Gothic arch, the remains of castles, and abbeys. They are consecrated by time; and almost deserve the veneration we pay to the works of nature itself." In this passage the link between the building and nature is weak, but in another place Gilpin makes the association much stronger when he says that the ruins of abbeys, ''being naturalized to the soil, might indeed, without much impropriety, be classed among it's [sic] natural beauties" (57).
When the poet in Epipsychidion describes the isle as "an isle under Ionian skies, / Beautiful as a wreck of Paradise" (422-23), we notice that there are some distinctive, picturesque elements in the poem. Consider the following lines, for example:
And, for the harbours are not safe and good,
This land would have remained a solitude
But for some pastoral people native there,
Who from the Elysian, clear, and golden air
Draw the last spirit of the age of gold,
Simple and spirited; innocent and bold. (424-29)
The picturesque, from what I understand of it, is a combination of the gothic, naturalistic, and the pastoral: gothic in that there are ruins, naturalistic in that the ruins create the impression that they are becoming part of the naturalistic landscape, and pastoral because the ruins are always in the country. Shelley seems to be combining picturesque elements with his historical imagination, combining the contemporary interests in the picturesque with his interest in Greek antiquity.
I noticed that there are two articles on the gothic and the picturesque listed in the MLA Bibliography, so there is plenty of additional research that can be done in this area. Here are the two articles:
Rowanchild, Anira. "'Everything Done for Effect': Georgic, Gothic and Picturesque in Anne Lister's Self-Production." Women's Writing. 7.1 (2000): 89-104.
Charlesworth, Michael. "The Ruined Abbey: Picturesque and Gothic Values." The Politics of the Picturesque: Literature, Landscape and Aesthetics since 1770. Cambridge: Cambridge UP. 62-80.
Bright, Michael H. "The Pleasure-House in Shelley's 'Epipsychidion.'" American Notes and Queries 17 (1978): 55-57.