reflection of rainbow
蜺 centers on a rainbow's reflection, a secondary or mirrored rainbow. The meaning is narrow and visual, tied to an atmospheric phenomenon.
蜺 combines 虫 (insect/creature) with 兒, which likely contributes the sound. The connection to a rainbow reflection is not obvious from the components, and the exact historical development is uncertain.
The insect radical 虫 beside 兒 suggests a small, curved creature. Picture a tiny insect-shaped arc of light mirroring the main rainbow: 蜺 is the reflection of a rainbow.
For ゲイ, imagine a gayly colored secondary rainbow: gay -> ゲイ, and the insect-like arc reflects the main rainbow's colors.