mortar, hand mill
磑 centers on a grinding tool: a mortar or hand mill used to crush or pulverize substances.
磑 combines 石 (stone) with 豈, which likely contributes the sound. The stone radical indicates a stone tool, fitting the mortar or mill meaning.
A stone (石) tool with a complex grinding mechanism (豈) is a mortar or hand mill.
For ガイ, imagine a guy using a hand mill to grind grain: guy -> ガイ, and the stone mill crushes it.