Most commuters at Gatwick are Captains with reasonably high seniority, so they can be pretty sure of the type of roster they will get. They can also afford to book with Easy etc. if staff travel goes wrong, or flight times don't suit.
It costs £40-£50, inc tax, return to most places in Europe with staff travel, 6 months to qualify.
Staff travel is best from places that fly to both Heathrow and Gatwick with many daily flights. These are limited to places like Geneva, and Nice.
There is a shuttle linking LGW to LHR but it's about £16 and no staff discount.
Lots of long haul FO's commute.