And they would have to change trains as stations like Hendon and Mill Hill don't have a direct service to Gatwick only a direct service to Luton
True, but relatively easily done at City Thameslink or Farringdon with no change of platform needed. It is as is easy/difficult as transferring to a shuttle bus at Luton Airport Parkway.
Whilst clearly not as convenient as LTN, LGW is
relatively easy for train travellers from Hendon, Mill Hill, etc., compared to, say, LCY, LHR, SEN or STN.