I eat HDR and SSAO for breakfast! It's the reason for using gamma at all that I don't understand, though this slide cleared it up a bit.
Using gamma space textures are good for precision reasons. This way you get more precision where you need it.(for dark values)
Using gamma space for monitors is different story. More reading. http://http.developer.nvidia.com/GPUGems3/gpugems3_ch24.html
About SSAO I find these papers really interesting.http://graphics.cs.williams.edu/papers/SAOHPG12/http://kayru.org/articles/dssdo/
SAO give better performance with really high quality with same inputs requirments than uncharted ssao.
On other hand dssdo is propably only feasible way to get some shadows for all those deferred points lights that you can throw nowadays.