Agree with Davidgordon that on a service such as the Gatwick Express staff shortages as a reason for cancellation is really really poor.
I use the Heathrow Express weekly to LHR and whilst i’ve never had a cancellation due to staff shortages i’ve encountered many a delay due to signal failures, broken down trains etc etc. I’ve learned to never rely on the last possible scheduled service to get me to the airport just in time for my flight. I always make sure I take an earlier service just in case. I also check the Heathrow Express website/Twitter feed just before leaving home so I can take my alternative route (the tube sob) if need be.