What major characteristic of Romanticism is evident in the first 20 lines of the poem "Elegy Written in a Country Churchyard"?

Although Thomas Gray's "Elegy Written in a Country Churchyard" is not considered to be part of the Romantic canon, its first twenty lines do advance some classic Romantic themes. Most notably, the first twenty lines of Gray's poem describe the kind of isolated world of natural beauty favored by most Romantics.
Though the first twenty lines develop this characteristic, it's pretty completely developed in the first stanza:

The curfew tolls the knell of parting day, 
         The lowing herd wind slowly o'er the lea, 
The plowman homeward plods his weary way, 
         And leaves the world to darkness and to me. (1-4)

Already in this first stanza, we can see that Gray is setting his poem within a world of pastoral beauty. With a wandering herd, an open grassland (the definition of a "lea") and a plowman, Gray skillfully develops a world of natural beauty and hard-working farmers. Moreover, since the world is left to the speaker of the poem, it's suggested that he is alone in this picturesque natural setting. A beautiful and isolated natural setting is a typical characteristic of Romantic literature, as it cultivates an ideal place for the Romantic imagination to become manifest. By using the first twenty lines of his poem to develop this setting, Gray also utilizes a feature that was to become a key part of the Romantic aesthetic.
https://www.owleyes.org/text/elegy-written/read/text-poem