toothless
齳 describes the state of having no teeth; toothless.
齳 combines 齒 (tooth) with 軍, which likely provides the sound. The exact historical development is uncertain.
The left side 齒 means tooth, and the right side 軍 looks like an army vehicle. Imagine an army tank rolling over a set of teeth, leaving them completely toothless.
For グン, picture a toothless old soldier in the army (軍, ぐん) grinning: 軍 -> グン, and his toothless smile matches 齳.
—