You would answer "(Es)geht so" if you are not feeling great or very well, but on the other hand not really bad either. It's something in between, like when you had an exhausting workig day and you are in need of a relaxing evening.
Usually the other person will ask you then, what's bothering you.

I believe "so so" means something in that context?

Ines

"It is our choices, that show what we truly are, far more than our abilities." (Albus Dumbledore, CoS)