It is not a setting in the UI.
If you set your project to use linear color settings, you must set the textures to use the linear color settings. It is a boolean which you pass when we create a new Texture2D:
public Texture2D(int width, int height, TextureFormat format, bool mipmap, bool linear);
Textures are created in the BundledMaterial class. It is important for the properties texture to be flagged as linear if the project is using the linear color space.