The verb G. {urtha- >>} urna- “blaze, burn (intr.)” appeared in the Gnomish Lexicon of the 1910s (GL/75), probably derived from the early root ᴱ√URU which had several fire-related derivatives (QL/98).
Neo-Sindarin: For purposes of Neo-Sindarin, I think this verb can be adapted as ᴺS. orna- from the later root √UR, and it can be used for “burn” in general (both transitive and intransitive) since the other transitive Gnomish “burn” words like G. gusta- cannot be easily adapted.
A verb in the Gnomish Lexicon of the 1910s glossed “work, wreak, fashion, make, devise”, which Tolkien indicated was related to G. ûr “smith” and G. {aur >>} aurin “made, wrought, fashioned” (GL/75).