precipice, shore, bank, surname, cliff
厓 centers on a steep edge where land meets water or drops off: a cliff, precipice, or shore. The surname sense is a derived use of the same character.
厓 combines 厂 (cliff) with 圭, which likely contributes the sound. The character originally depicted a steep cliff or riverbank.
The cliff 厂 encloses 圭, like a steep precipice rising above a shore. Picture a sheer cliff face with a narrow bank at its base.
For ガイ, imagine a guy standing on the cliff edge: guy -> ガイ, and he looks down at the shore below.