I think one of the main problems with busy single runway airports like Gatwick/Stansted is that for business continuity the major airlines need a contingency in the event that the single runway becomes unavailable, so they are not going to create a major base/hub for this reason. Gatwick's Northern runway is not really suitable for this purpose as it is capacity limited and a visual operation. Yes it's Ok to use at night durng planned routine main runway maintenance (provided weather meets visual operating limits) but to use unplanned during busy periods is not ideal. Having said that when Heathrow is on single runway operations then this isn't ideal either, but Heathrow operates to near capacity all day unlike Gatwick, so not surprising.
Gatwick can achieve between 800-900 daily movements in summer months, partly because there are now less wide bodied movements than a few years ago which enables increased runway capacity . Also it has a more generous night movement quota compared to Heathrow which means many more movements at night.
Runway declared capacity and stats for Gatwick and other airports are on the Airport Co-ordination web site.