If it's the liftgate window, you can do it easily with a piece of string. Put the gasket on the window. Now cut a piece of cord like sash cord, not too thin, to a length that is a few feet longer than the total length of the outside groove in the gasket. Now thread the string into the groove, so that both ends meet at one spot, or overlap a little. Now open the liftgate and lay the glass on top of the hole, and get inside. With your hands, start pulling the gasket over the lip of the gate, and once it's well started, just very carefully pull on the string and it will pull the rest of the gasket over the lip as it goes around.
You use the same procedure on the side quarter windows, but of course you can't lay the glass down, so you'll either need a helper, or need to manually seat the bottom edge before you pull the string.