I can assure you Postprocess AO and SSGI would not work together, you can use either one SSAO (Post-processing AO) or SSGI. SSGI kinda more accurate and doing its own AO and costly too. if you wanna combine them together that you need to change from the source. Even if you are working with baked lighting does SSGI replaces that. But I am still unsure about the wrong with AO from G-buffer. I had the same issue once before I guess, but I cannot really memorize what I have done, I would suggest you port the project to another engine version if you are arranged with your level assets.
