Gothic texts are defined through their use of the horrific, grotesque, and dark imagery. While this does not define a definitive use of the term Gothic, some texts can be seen as Gothic based upon the simple images depicted in the text.
P. B. Shelley's poem Mont Blanc does have some aspects of Gothic intertwined with its rich and highly descriptive natural imagery. Therefore, by choosing words which depict the Gothic, one can define certain elements of the poem as containing Gothic characteristics.
The everlasting universe of things
Flows through the mind, and rolls its rapid waves,
Now dark--now glittering-no", reflecting gloom.
Here, the mind of Shelley can be seen as one which is dark and conflicted. (Think about the characters seen in Poe's writing, Poe being the father of the Gothic, who are dark and demented.)