Seeing which, Gouie spoke...
Seeing that aforementioned thing, Gouie spoke...
There was a car broken down on the shoulder ahead being hauled up onto
a tow truck, seeing which, he moved into the outer lane.
The use of which in this way is related to the following use, with which you are no doubt more familiar:
There was a stew simmering on the burner, the smell of which made him
I suspect that what is confusing you in the original is the participle, in light of which, I offer this paraphrase:
Keo recognized the scent of a black man and tried to raise his head
high enough to bite him, in light of which, Gouie spoke in the
hippopotamus language, which he had learned from his grandfather...
You're familiar with the use of which as the object of a preposition (the smell of which, in light of which) and now you understand that which can also be the object of a verb. It is a somewhat dated use.
He felt the weight of the shillings|silver dollars in his pocket, having which, he decided to stop by the pub|saloon for a beer.