Gnosis is similar to the Polynesian concept of mana, or the Eastern concept of chi. While all creatures have some spiritual component, Garou can consciously control and use it. Garou are quite literally half-spirit already, and Gnosis reflects how connected they are to their other half.
In the Storyteller System, Gnosis is ranked with dots, as well as boxes. The dots are permanent Gnosis while the boxes represent points of temporary Gnosis. A Garou character may not have more temporary points of Gnosis than permanent dots of Gnosis.
Gnosis is a renewable, but dwindling, resource. Garou can regain it at sacred sites, by meditating, or bargaining with spirits. Sacred sites provide Gnosis, but if they become defiled, it can corrupt the Garou, or there simply may be no more available. The water analogy works well here. They need water, but they need to protect their sources because drinking tainted water may make them sick, and if they aren't careful, their well may go dry.