I think there may be a glitch in gdebugger where it is reporting MB as GB

That sounds possible. What resolution are you using? At ~1080p with your described setup, and regular depth buffers, you would be somewhere around the ~280MiB mark...

Why do you need a multisampled G-Buffer and a non-multisampled G-buffer?

I think there may be a glitch in gdebugger where it is reporting MB as GB

That sounds about right.
What resolution are you using? At ~1080p with your described setup, and regular depth buffers, you would be somewhere around the ~280MiB mark...

Why do you need a multisampled G-Buffer and a non-multisampled G-buffer?