parapet
堄 centers on a defensive wall structure: a parapet, a low protective wall along a roof or rampart.
堄 combines 土 (earth) with 兒, which likely contributes the sound. The earth radical suggests a structure made of earth or stone, fitting the parapet meaning.
The left side 土 is earth or soil, and the right side 兒 looks like a child. Imagine a child building a low wall out of earth to protect a toy castle: that earthen wall is a parapet.
For ゲイ, picture a guard leaning against the parapet and giving a friendly wave: 'Hey!' The guard's 'Hey!' sounds like ゲイ and helps you remember the reading.