In the context of my experience, gland water would refer to seal flush or packing flush water. If the pump has a mechanical seal, the outer plate of the seal is called a gland. If water is injected into the seal through a connection on the gland plate, it would be done for the purpose of flushing the seal. Seal flush can serve a number of purposes. It can cool the seal if the flush water is cooler than the pump product. It can carry away the heat generated by the seal faces if the flush water is the same as the pump product. It can keep contaminants out of the seal chamber if the flush water is cleaner than the pump product.
The term gland water could also be used for water injected into shaft packing to cool and lubricate the packing. Packing is much less common than mechanical seals, but we still use packing in cooling water, well water and some other water services.
Johnny Pellin