No Departure lounge at LGW for Premium passengers but this would be the same if you were flying with any other charter airline in an upgraded cabin.
You will have your own dedicated check-in and be invited to board first but no special lounge!!
I flew with Virgin Atlantic last year and as one of their pax you can pay to go into their lounge at LGW (£20 approx).
Not sure if you can do similar when flying MON, ask ground staff at checkin!!!