Originally Posted by
NWIFlyer
Assuming the OP hasn't already paid for the coach, asking again on arrival at LHR may well save a great deal of aggravation later!
Anytime I've been re-routed from LHR to LGW, I had to pay for the coach upfront.
Given that the OP probably won't have a lot of time to faff about, I would recommend just buying the ticket, travelling to Gatwick, and submitting the ticket with a claim for reimbursement after the trip is completed.