The GI cache is an internal data cache that the Unity Editor uses to store intermediate files when precomputing lighting data for Realtime Global Illumination (deprecated), lightmaps, Light Probes, and Reflection Probes. By storing this data in a cache, Unity can speed up subsequent precomputations.
The cache is shared between all Unity projects on the computer, so projects with the same content and lightmapping backend can share the files.
You can manage the size, location, and compression of the cache using the GI Cache preferences. For more information, see the Preferences documentation.
The Unity Editor uses the GI Cache when it automatically precomputes lighting data. To enable the GI Cache, open the Lighting window (menu: Window > Lighting Settings) and enable the Auto option next to the build button. This makes light data precomputations automatic, and stores data in the GI Cache.
In the Lighting window, you can clear the baked data in a Scene (disable the Auto option and choose Clear Baked Data from the Build button drop-down menu). This does not clear the GI Cache, because this would increase bake time afterwards.
You can share the GiCache folder among different machines. This can make your lighting build faster, because the files are downloaded from the GiCache folder instead of computed locally. Note that the build process isn’t optimized for slow network-attached storage (NAS), so test if your bake times are severely affected before moving the cache to NAS.