"Goo" is used to describe a substance that's not quite solid because it has too much liquid to be completely solid. Its consistency is like a batch of gelatin that has too much water to become firmly gelled.

It might be related to the adjective "goopy" which describes something the texture of soft mud; in other words, there is a lot of liquid, but still a bit of structure. Sadly here it describes the insides of his now dead friend. "Goo" and "goopy" are words kids (children) use a lot to describe something half-liquid and half-solid that is unpleasant to the touch and slightly revolting. I am not sure you will find them in a dictionary. But they work well given the unpleasantness of the situation.