If you’re bringing a bike, verify cycle policies on trains; some lines restrict non-folding bikes

https://www.scribd.com/document/1074920514/Getting-to-Heathrow-from-Gatwick-Easiest-Inter-Airport-Transfers-178501

If you’re bringing a bike, verify cycle policies on trains; some lines restrict non-folding bikes, so consider airport buses with luggage racks instead.