It does not necessarily mean that they will become unregulated - while that would be the optimal solution from an economic point of view, I do not believe that would be feasible for a number of years yet.
The sale of Gatwick is great news for the industry, as in the short term it will indeed permit development of the airport as a competitor to other airports, hopefully raising standards of service.
While Virgin ownership of Gatwick might be viewed to be negative for other carriers, it is not entirely uncommon for airlines to have significant ownership interests in, or close ties to airports - many in the Far East are an example.
They are unlikely to be permitted to charge differing access charges to different airlines, but they would be able to enhance their service standards to the detriment of others. However, with competition among airports, those airlines are free to move elsewhere - overall, I cannot see any particular negative impact of Virgin themselves owning the airport.