The union jack flag will go to be replaced by the easyjet logo, apart from that I think nothing else will change, maybe cheaper flights once they settle in.
Good for Gibraltar without a doubt (where the GB in GB Airways comes from) because we need the competition on this london-Gib route. Tickets from London to Gibraltar are pretty expensive most of the time as the only other carrier flying it is Monarch Airlines.
Easyjet will hopefully sell seats at a competitive price.
Also, are you aware that British Airways has announced that they are planning to introduce routes on most of the GB Airways ones that are being taken over?
you can't guarantee Easyjet will keep all or any routes; sometimes companies take over rivals just to close down bits of their business that are making for too much competition. You'll probably need to wait and see exactly what Easyjet plans to do with it.
I would expect GB airways aeroplanes will go - that is the Airbus A320 and A321, replaced by the smaller A319 of Easyjet. The cabin will be different too, Easyjet of course are all one class whereas GB airways being part of BA had business and all that goes with it - access to BA miles and club privelidges etc, if that is how you are accostomed to trravelling!