there were bike racks in south terminal at one time but not sure if they are still there due to current refurbs and sussex plod view that bikes are a security risk.
nothing you leave anywhere at gatwick while you work there is secure in the experience of members of my family.
BAA used to have plans for workers sustainable transport to and from the airport, which they developed in tandem with Crawley Borough Council b ut who knows what GIP the new owners are about. The current Gatwick website doesn't even mention cycling.