As previously mentioned, it's the venting. What's happening is that (sometimes) the drain line is completely filled when you flush and as a result isn't allowing air to pass over the flushed water in the drain line. If this "plug" of water passes where your sink connected to the main drain, before it passes the vent, the partial vacuum behind the plug will try to suck air in past the trap in your sink. Hence the gurgling. Happens in our camp too. When I redo the bathroom I'll make it proper. You'll have to hire a plumber.
As long as everything works I wouldn't worry about it.