Oops, I’m sorry, @emyoulation, I realized as soon as I replied that I had misunderstood you. (Of course you would not be asking how to export kml files that you yourself had already imported!) I agree it would be nice if there were a Tool or Utility in Gramps to export selected places as a kml file. Until such a tool or utility is created, it might be possible to use XSLT to transform place object data in the Gramps xml file into a kml file – straightforward but, alas, never simple.
You mentioned that your polygon kml file did nothing when you added it in Gramps. I created one and it shows up in the Geography view like this:
Since there is no pin, I have to zoom in pretty far before it appears on the screen. That’s obviously not very useful. I guess I could also add a place for the centerpoint, so that it gets a pin. Or maybe there’s a way I can add one within the kml file; I don’t know enough about it.