A verb for “die” in Early Noldorin Grammar of the 1920s with present form gwardh indicating vowel gradation (PE13/132), so that it was likely based on the early root ᴱ√GWṚÐṚ (QL/104) with a/u variations due to the different developments of long syllabic ṝ vs short ṛ.
Conceptual Development: The verb G. gor-“die” appeared in the Gnomish Lexicon of the 1910s related to gurthu “death” (GL/41, 43), thus also based on the early root ᴱ√GWṚÐṚ (QL/104).
Neo-Sindarin: S. gurth “death” survived in Tolkien’s later writings, so in theory the Gnomish verb gor- could be salvaged as a derivative of the later root √ÑGUR “die”. However, gor- already serves various functions in Sindarin, so I’d stick with the later verbs fir- and [N.] gwanna- for “to die”.
A verb for “to die” in Late Notes on Verb Structure (LVS) from 1969 (PE22/152), clearly based on the root √KWAL having to do with pain and death (PE18/91, 103; Ety/KWAL). As such, I would use this verb for undesirable or painful death, as opposed to fir- “to die (a natural or peaceful death)”.
Conceptual Development: ᴱQ. qal- meant “die” in Early Qenya Word-lists of the 1920s (PE16/134), and the root √KWAL had a long history of connection to death and pain in Tolkien’s writings.