7pm isn't that late though, the flights will be departing at 7:25pm or 7:30pm so it would only be an extra hour or two of operations. Also, it stays open to a similar time on Friday, Sunday, Monday and possibly Thursday during the summer already to accommodate Ryanair flights to/from London, so it would only be an extra three or four days a week and it would be during the busier period of the year.
However, you're right, it's hopeless for outbound connections from Gatwick.
Separately, I see Knock has issued an unusually muted response to the announcement of capital funding for the proposed apron and runway improvement works. I suspect they think it is a pre-election stunt and will believe that the funding will actually be provided when they see it.