The spelling "Ghyll" is first used in the edition of 1820 in the text. In the note to that edition it remains "gill". In 1827 the spelling in the note was "ghyll."—Ed.
Compare Dr. John Brown:
Not a passing breeze
Sigh'd to the grove, which in the midnight air
Stood motionless, and in the peaceful floods
Inverted hung.
and see
A to page 31.—Ed.