The gentian is a group of plants, with approximately 180 varieties.
The gentian described by William Cullen Bryant in "To The Fringed Gentian" sounds like the Autumn Gentian.
The bulk of Bryant's poem is an extensive description of the gentian.
It blossoms in the autumn; it is colored sky blue. It does not blossom in springtime like violets or columbines. Rather, it blossoms late, when "woods are bare and birds are flown" away, and frost and shorter days signal that the year is almost over. That is when the gentian looks "through its fringes to the sky."
In the last stanza, Bryant uses the gentian as a metaphor. Just as the gentian blossoms when the warm weather is nearly over, he hopes that when his own "hour of death" approaches, hope will blossom within his heart and he will look to heaven as he departs from this world. In other words, he prays that he will remain cheerful and hopeful even at the end of his life.