To be fair to them, planning the timetable is done by a planning team who will have had to do their best within the constrains of the reduced capacity.
The reality is, Gatwick Express brings in money and therefore is a priority for the DfT and therefore Southern. With the way that pathing works, you’re going to need to remove more than one non-stop GW to fit a stopping train in.
At the end of the day, if everyone at Redhill line stations managed to get onboard, then it sounds like the planning team did a good job, even if that resulted in a less satisfactory journey for you on this occasion.