Although it looks like a gasket, it is not. It is actually the means to mount a window panel in the hole. Look closely and you'll see that is has a groove around the outside that allows it to slip over the edge of the hole in the metal door, holding the rubber in place. Likewise it has another groove on the inner edge, and the window (cut to just the right side) is slipped into this groove, holding it in place. Fitting them together takes a little practice and patience. You slip the window into the groove on one end, then use a small flat tool like a screwdriver blade. You slip the blade tip between the window and the rubber where the rubber is just outside the window, then slide it along the edge to pull more of the rubber to outside the window. Go all around until all the window edge is into the groove. This is VERY similar to what is done to install automobile windshields.
It name? Don't really know. How about window mounting gasket? Window retainer? Window mounting ring?