Gamma is a power function, filmic contrast looks are simple sigmoid functions (classic S-shaped curves you apply to increase contrast) “mapped” on filmic (18%gray as mid gray).
Gamma 2.2 is already and always applied as a display transform (Display tab sRGB, Rec709…), the gamma slider is in blender just if you need an extra gamma correction and should not be used.
Another thing that could make your interior render darker is clipping expecially indirect at very low values. In internet there are old tutorials about “noise reduction” that use to promote clipping as a holy grail in noise reduction but they don’t explain how much destructive clipping is and how it works.
Told this, I think we’re going blind in this thread tryin to guess where your problem is.
I keep my guess on lighting setup, but if you don’t post at least a pic we’re going nowhere.