It's probably not a permitted route, but a totally sure step-free way
(it involves walking across a level crossing!) would be to take the
xx03 Reading train from Gatwick to Wokingham (journey time 66 mins),
then change for the xx26 Waterloo train from Wokingham to Richmond
(journey time 49 mins).

Actually, I've just checked, and it is indeed not a permitted route -
so you'd either need two tickets or to feign ignorance to the FGW
guard.

James