pour
漑 centers on pouring liquid, especially water, over something.
漑 combines 氵 (water) with a phonetic element that historically contributed the sound and the idea of pouring. The exact origin of the right-side component is uncertain.
The left side is water 氵, and the right side looks like a container with a spout. Picture water being poured from that container.
For ガイ, imagine a guy pouring water from a large container: guy -> ガイ, and the water flows out.