rainbow
霓 centers on the atmospheric arc of a rainbow, specifically the secondary, fainter bow sometimes distinguished from the primary rainbow.
霓 combines 雨 (rain) with 兒, which likely contributes the sound and relates to the idea of a secondary rainbow appearing after rain.
Rain 雨 falls, and beneath it appears a child-like 兒 arc of colors: the secondary rainbow 霓.
For ゲイ, imagine a rainbow appearing after a storm, and someone exclaims 'Gay!' (ゲイ) at its beauty.
rainbow