If I was going to work for either it would be B-Med, I have alot of mates there and it is supposed to be reasnoble there. GB is really bad, a friend of mine left there having only been there 3 months - she was an experienced flyer and worked for a few airlines before but said it was the worst place ever.
I have heard a rumour about B-Med though, that their franchise is up for renewal sometime soon and it does not look as if it will be renewed. Therefore I don't know what the future holds with them.
If you want an airline out of Gatwick then why not try Astraeus, Excel or Esayjet?
Gaz