Koko and her sign language is actually quite remarkable. She not only used words in proper context, indicating conceptual understanding, she also would combine two words to express something she didn't already have a word/sign for. She acquired a very substantial vocabulary/lexicon of signs, and could express herself with pretty amazing clarity, especially in her later years. She also taught her offspring. What this says to me is that the cognitive basis for language exists before the physical processes develop. Unless, of course, it was our evolution of grunts and simple name gestures within our social context combined with our ever more upright stance and neck configuration which allowed us more refined control of the sounds we make, leading to defined verbal language, vocabulary and syntax which overshadowed any gesturing except during hunting when silence was needed. Only later would a more comprehensive gestural language be developed. And in this paradigm we humans imparted language to primates long before they had symbolic gestural communications, and that it required us and our pathway to be able to introduce verbal language of any form to them. And in doing so we effectively "violated the prime directive" by altering the course of their natural development. I guess the next stage of the experiment, were it not for contagions, would be to release signing great apes back into wild populations to see if this spread through different groups, and if they would find it an adaptation that betters their chances of survival or expands their capacity for abstraction.
It is of interest to note that not only gorillas have learned this, but also chimps and orangutans, so there's some underlying predisposition for language amongst all of us.