Monarch had the advantage of operating 150+ seat aircraft and was heavily focussed on leisure flying between the UK and the Iberian peninsula
Eastern fly smaller aircraft to Gibraltar, are business focussed, with zero historic presence between the UK and Spain
I understand that 9 months ago, Eastern flying to Gibraltar was a good idea when everywhere else was closed and everybody was working at home. Those market conditions have now stopped.
If routes from SOU and BHX to Gibraltar are to work, it will need to be on a 150+ seat aircraft operated by an airline with a brand widely recognised by leisure travellers