After above discussion about GH's size, I went back and watched that scene again. He/it actually is pretty big, look at the scale of how small Coulson looks standing next to the tank. Even if you take into account foreground perspective, GH is still, for lack of a better word, giant.
I was previously for no good reason leaning towards something human, even super-soldiery, because GH sure has some pretty normal looking human features like teeth, nipples, ears, pelvis, and so on. Even the nails could be explained as continued growth.
Now the very scale issue I mentioned above makes me second guess. Really looks human though, just biggg human.
Anyway, take a look at the symbols appearing on GH. Two prominent ones on his left breast, one is very runic-looking and the other looks like a modified swastika. (Not saying it is a Nazi or any known swastika, just describing the appearance).
Below those two are a vertical string of smaller characters on the left abdomen. I can't see them that well, they almost look like Asian hanzi/kanji characters, but if if you could find them out to be runes, I'm cool with that.
There is another symbol very briefly seen on his/it's left arm.
And also, one on its right breast better seen/lit in the first shot, disappointingly not so much in the shot that shows the right breast better.
Any identifiable correlation between any of those symbols, frost giants, Asgardian runes, or such? Guess I'm going to have to re-watch CA:TFA and both Thor films to find or eliminate that idea...