This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
鑈 combines the metal radical 金 with the phonetic component 爾. The exact historical meaning is uncertain, but it likely referred to a metal object or tool.
The left side is metal 金, and the right side 爾 looks like a complex tool. Imagine an old, mysterious metal implement whose purpose is now forgotten.
For ゲイ, picture a gate made of this mysterious metal: gate -> ゲイ, and the gate is forged from the metal 金 on the left.