Gnomish
dâd
noun. grandfather
nâd
noun. grandfather
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 noun for “grandfather” in the Gnomish Lexicon of the 1910s with variants G. dâd and nâd, the latter related to G. nathon “father” (GL/29, 59).
Neo-Sindarin: As we have no other attested alternatives, I would use ᴺS. dâd for “grandfather” in Neo-Sindarin, but would avoid nâd since in later writings Tolkien use naneth for “mother” (Ety/NAN).