From the FGW website :

17:26 Gatwick Airport to Reading due 18:52
This train has been cancelled. This is due to resourcing difficulties..

This service is another one whose reliability isn't the best. I would say once a fortnight it is cancelled. I am sure someone will know, but I had read that this is an expensive train to operate. Also, there's a 1747 Redhill to Reading service which can cover the cancelled train