It could be down to the fact that you spammed your site with gsa ser - I wouldn't use any link that creates direct to my site, since i feel there's too a high a percentage of those links are on the list of disavowed links, but that's just me. Google doesn't need to know every link that gsa ser creates - it only needs to know a certain percentage and you can bet that they do. If your site's backlink profile meets this percentage then it's bye bye - Or at least that's what i would expect.
But, and this is just a theory that i haven't tested properly yet - I have seen a few threads like this pop up in the last couple of weeks - It could be a case that google is now treating any backlink profile with too high a percentage of web 2.0s and / or wordpress links as suspect sites and filtering their rankings.
Again, you don't have to take a word i say as truth since i haven't tested it fully, but im sure google know that the vast majority of blackhatters have been using wordpress pbns and web 2.0s to rank sites, and maybe some gsa spam.
A natural website's backlink profile wouldn't consist of more than a certain percentage of wordpress and web 2.0 links, so why not filter any sites who go over a certain percentage of these links?
Ye there might be innocent casualties on the way, but the majority of blackhatters are gonna struggle to rank their sites easily. That's the way id be looking at it if i was matt cutts anyway. If it's not happening now, i wouldn't be too surprised if it happens soon.
You said the content linking to your site was good quality, so the only other thing i could think of if it's none of the above is the content on your site. You know panda 4.0 was rolled out recently as well ye? Ignoring keyword density, how many pages does your site have - i.e is it being penalized for being a thin site. And how long are your posts? - i.e is it being penalized for being a thin site - short, unstructured posts = low quality content, is what im seeing since the latest panda update. 800 words + is what im personally doing now.
Just my 2 cents anyway.