Combine global illumination with Final Gather to achieve the most physically accurate illumination results, which are particularly good for interior architectural shots that require the effect of the light contribution from exterior and interior light sources.
To seamlessly turn final gather on and off with global illumination, your scene must be modelled in a physically plausible way for both of these lighting effects. For example, lights should have roughly identical values for Color (direct light) and Energy (photons) attributes.
To combine global illumination with final gather (guidelines)
See Create a Maya light source for more information.