Middle Primitive Elvish
gas
root. yawn, gape
gassā
noun. hole, gap
tundu
noun. hole
yag
root. yawn, gape
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A root in The Etymologies of the 1930s glossed “yawn, gape” with several derivatives such as ᴹQ. assa/N. gas “hole” and N. gaw “void” (Ety/GAS). The names based on this root like N. Belegast and ᴹQ. Ilmen-assa do not appear in later versions of the legendarium, so the eventual fate of this root is uncertain, especially given that there are other roots of similar meaning like ᴹ√YAG “yawn, gape” (Ety/YAG) that do reappear later (Let/383; PE17/35, 42).
Neo-Eldarin: For purposes of Neo-Eldarin, derivatives of this root are among the best attested words for “hole”, so it’s probably best to retain ᴹ√GAS and its derivatives in Neo-Eldarin writing.