Gatwick uses a different bidding system to LHR. Carmen (the Gatwick system) still relies on seniority to determine rosters but doesn't have the same 'gradient' as Bidline does at LHR. i.e. - everything else being equal, junior guys have better rosters at Gatwick.
With Carmen, you submit a monthly bid with various preferences. These could be days off, early day trips, late starts, tours, no 4-sector days, whatever really. Carmen will look at everyone's bid and try to achieve 40% satisfaction for the most junior pilot, increasing with seniority. The big bonus with Carmen is that anybody, regardless of seniority can guarantee (99.999%!) one 1-4 day block off, even over a weekend. The downside is that to do that you must only bid for that one block of days, so the rest of your month will be a lottery. In short, it's a pretty good system for junior guys.
Gatwick has very few nightstop destinations (6 now), so if you want to do longer tours, you stand a better chance of getting them at LHR.