gums
齦 refers specifically to the gums, the flesh surrounding the teeth.
齦 combines 齒 (tooth) with 艮, which likely contributes the sound. The character is a phono-semantic compound for gums.
The left side 齒 means tooth, and the right side 艮 looks like a firm, fleshy base. Picture the gums as the foundation holding the teeth in place.
For ギン, imagine a grinning mouth showing healthy gums: grin -> ギン, and the gums are visible when you grin.