GI works for the sun only, it doesn't work for individual lights!
It really doesn't add too much, the differences are minor, they're supposed to be, it's light bouncing not light casting, but their there.
You can although, really pump up the GI strengh in the TOD.
ON: (1 = max)
Values of 0.3/0.4 usually create good results, it really depends on your ToD, how bright the sun is, etc, play around.
VizAreas have a IgnoreGI on and off flag, but again, this only works if you have sun inside your viz-area.
To fake GI in interiors, we used manually placed deferred lights, their cheap so you can use a lot of them.