This is caused by revenue management for LGW. The numpties zero out all of the cheapies to ensure that you go to easyJet, Ryanair or LHR if you do not book a long way in advance. This helps the revenue at LGW by ensuring that both passengers who book a full fare in a month pay a lot more than the xxx hundred pax that wanted to fill the now empty seats at a reasonable fee.

sometimes, the policies at Gatwick are designed to generate a large profit, painted in red with a large minus sign in front of them.