Gunnar died of liver failure. Chances are it was some sort of poison - either he ate a mouse or some bugs that came out of our neighbor's house after the exterminator sprayed, or a black widow bite (we have a lot of black widow spiders here).
We miss him terribly!
As sad as this is...it is also good news. With the rampant spread of disease, this is the best news possible.

Again..sorry for your loss.