Most, if not all of the lyrics in Old Norse are actually taken from heathen literature: for example, the chorus lines in the song "Donar’s Oak" are actually verses four and five of Grímnismál, a poem of the Elder Edda.
Snorri's knowledge of Níðhöggr seems to come from two of the Eddic poems: Grímnismál and Völuspá.
Regarding this stanza, scholar Andy Orchard comments that the name Gjallarhorn may here mean "horn of the river Gjöll" as "Gjöll is the name of one of the rivers of the Underworld, whence much wisdom is held to derive", but notes that in the poem Grímnismál, Heimdallr is said to drink fine mead in his heavenly home Himinbjörg.
Gulltoppr is mentioned in a list of horses in the Poetic Edda poem Grímnismál and in Nafnaþulur section of the Prose Edda.
In Grímnismál, Odin and Frigg are both sitting in Hliðskjálf when they see their foster sons Agnarr and Geirröðr, one living in a cave with a giantess and the other a king.