Since January at least 3 A320 have been sent to Gatwick each weekend to help cover the extra weekend flights both scheduled and chartered. They have returned Sunday evenings. However with the summer schedules starting tomorrow they will probably staying longer.
As we near the peak summer season more A320s will probably be joining them. A320 regularly seem to swap between the bases nowadays with the exception of the G-GAT aircraft.
Gatwick based aircraft are also coming to the end of their winter maintenance programmes. At the moment the G-MID aircraft are also being reconfigured from 168 to 180 like the LHR aircraft. I think a couple have been done already.This will allow all A320 aircraft to serve both bases if necessary without causing scheduling issues regarding seat numbers. I believe the G-GAT aircraft are staying at 177 seats though.