The old Dan-Air operation at Gatwick was short-haul only, merged at a time when BA also had quite a number of short-haul ops from there, but nowadays there is a substantial long-haul operation out of Gatwick as well, with a semi-dedicated 777 fleet. Incidentally, BA bought Dan-Air for £1 (plus its huge debts), which gives you some idea of the value of that old network. There was a fair bit of acrimony at the time about the merger, so at an interview you might keep off the topic !
By no means all the destinations are just leisure ones there's no space for at Heathrow. Quite a number of points are served from both at high multi-day frequencies, such as Nice or Rome, both of which have substantial all-year business traffic, and traffic originating from the far end (not a common characteristic of leisure routes). There are also mainstream trunk domestic flights to Glasgow and Edinburgh. To some extent Gatwick serves a separate geographical area of London/South East, which can warrant this duplicate service to the busiest destinations. There's also a restart this year of a BA Gatwick flight to New York JFK.