It's been my experience that the luggage is left untouched at gatwick - naturally a couple of cases may be pulled aside, but generally the case owner would be taken with it and go through it together with a customs officer.
Coming back, if you don't have TSA-approved locks, expect missing locks and cable ties holding the case locked! be aware that they're thorough with their searches, and not very gentle, so if you've got anything remotely breakable, try to pack it in your hand luggage.