Re: 齧 vs 囓 vs 嚙

現代汉語詞典 -- 嚙 is standard; 齧 and 囓 are variants. The single meaning is “gnaw”; there is no Canto meaning of “grind (teeth)”.
廣州話正音字典 -- 嚙 and 囓 are variants; there is no 齧. There are 2 readings: ngaat6 for “grind (teeth)”; jit6 (variant: nip6) for “gnaw”.
廣州音字典 -- 齧 and 囓 are variants; there is no 嚙. Standard reading is ngit6; variant, ngaat6. The single meaning is “gnaw”; there is no “grind”.
LSHK -- 齧 has 3 readings: ngaat6, jit6 and ngit6. 嚙 and 囓 are variants with a single reading of ngit6.
CUHK (4 sources) -- 齧 is standard; 嚙 and 囓 are variants. Standard reading is jit6; variants, ngit6 and ngaat6. The single meaning is “gnaw”.
廣州話方言詞典 -- 扤 [ngaat6] for “grind”. 嚙 [colloquial: ngaat6; literary: ngit6] for “gnaw”.
廣州話普通話詞典 -- 嚙 [colloquial: ngaat6; no literary] for “gnaw”.

In short, 嚙 looks more like the standard form for “gnaw”. The readings of ngaat6 (colloquial) and ngit6 (literary) as suggested by 廣州話方言詞典 should be recommended.
As for “grind (teeth)” which is also read as ngaat6, 廣州話正音字典 suggests 嚙 (and variant 囓). 廣州話方言詞典 borrows 扤 for its identical pronunciation. Other dictionaries have 扤 for other meanings; they don’t have any word for “grind”.