I've been open to either airport, but actually I see three flights on that Sunday from LGW and two flights that day from LHR. The first one from LGW is too early to connect, and the last one is that late one I'm trying to avoid on the next day, so actually there are two "good flights" from LHR and only one "good" flight from LGW. Or if waiting till the next day (Monday) there are two other options from LGW, but the best flight timewise would again be from LHR.